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691 lines
32 KiB
Plaintext
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Developer's Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated: Wed Apr 17 01:12:20 EDT 2002
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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The most recent version of this document can be viewed at the
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postgreSQL Web site, http://www.PostgreSQL.org.
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_________________________________________________________________
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General Questions
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1.1) How do I get involved in PostgreSQL development?
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1.2) How do I add a feature or fix a bug?
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1.3) How do I download/update the current source tree?
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1.4) How do I test my changes?
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1.5) What tools are available for developers?
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1.6) What books are good for developers?
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1.7) What is configure all about?
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1.8) How do I add a new port?
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1.9) Why don't we use threads in the backend?
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1.10) How are RPM's packaged?
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1.11) How are CVS branches handled?
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1.12) Where can I get a copy of the SQL standards?
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Technical Questions
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2.1) How do I efficiently access information in tables from the
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backend code?
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2.2) Why are table, column, type, function, view names sometimes
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referenced as Name or NameData, and sometimes as char *?
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2.3) Why do we use Node and List to make data structures?
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2.4) I just added a field to a structure. What else should I do?
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2.5) Why do we use palloc() and pfree() to allocate memory?
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2.6) What is elog()?
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2.7) What is CommandCounterIncrement()?
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_________________________________________________________________
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General Questions
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1.1) How go I get involved in PostgreSQL development?
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This was written by Lamar Owen:
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2001-06-22
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What open source development process is used by the PostgreSQL team?
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Read HACKERS for six months (or a full release cycle, whichever is
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longer). Really. HACKERS _is_the process. The process is not well
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documented (AFAIK -- it may be somewhere that I am not aware of) --
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and it changes continually.
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What development environment (OS, system, compilers, etc) is required
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to develop code?
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Developers Corner on the website has links to this information. The
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distribution tarball itself includes all the extra tools and documents
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that go beyond a good Unix-like development environment. In general, a
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modern unix with a modern gcc, GNU make or equivalent, autoconf (of a
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particular version), and good working knowledge of those tools are
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required.
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What areas need support?
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The TODO list.
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You've made the first step, by finding and subscribing to HACKERS.
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Once you find an area to look at in the TODO, and have read the
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documentation on the internals, etc, then you check out a current
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CVS,write what you are going to write (keeping your CVS checkout up to
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date in the process), and make up a patch (as a context diff only) and
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send to the PATCHES list, prefereably.
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Discussion on the patch typically happens here. If the patch adds a
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major feature, it would be a good idea to talk about it first on the
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HACKERS list, in order to increase the chances of it being accepted,
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as well as toavoid duplication of effort. Note that experienced
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developers with a proven track record usually get the big jobs -- for
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more than one reason. Also note that PostgreSQL is highly portable --
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nonportable code will likely be dismissed out of hand.
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Once your contributions get accepted, things move from there.
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Typically, you would be added as a developer on the list on the
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website when one of the other developers recommends it. Membership on
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the steering committee is by invitation only, by the other steering
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committee members, from what I have gathered watching froma distance.
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I make these statements from having watched the process for over two
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years.
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To see a good example of how one goes about this, search the archives
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for the name 'Tom Lane' and see what his first post consisted of, and
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where he took things. In particular, note that this hasn't been _that_
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long ago -- and his bugfixing and general deep knowledge with this
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codebase is legendary. Take a few days to read after him. And pay
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special attention to both the sheer quantity as well as the
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painstaking quality of his work. Both are in high demand.
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1.2) How do I add a feature or fix a bug?
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The source code is over 350,000 lines. Many fixes/features are
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isolated to one specific area of the code. Others require knowledge of
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much of the source. If you are confused about where to start, ask the
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hackers list, and they will be glad to assess the complexity and give
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pointers on where to start.
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Another thing to keep in mind is that many fixes and features can be
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added with surprisingly little code. I often start by adding code,
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then looking at other areas in the code where similar things are done,
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and by the time I am finished, the patch is quite small and compact.
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When adding code, keep in mind that it should use the existing
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facilities in the source, for performance reasons and for simplicity.
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Often a review of existing code doing similar things is helpful.
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The usual process for source additions is:
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* Review the TODO list.
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* Discuss hackers the desirability of the fix/feature.
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* How should it behave in complex circumstances?
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* How should it be implemented?
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* Submit the patch to the patches list.
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* Answer email questions.
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* Wait for the patch to be applied.
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1.3) How do I download/update the current source tree?
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There are several ways to obtain the source tree. Occasional
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developers can just get the most recent source tree snapshot from
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ftp.postgresql.org. For regular developers, you can use CVS. CVS
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allows you to download the source tree, then occasionally update your
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copy of the source tree with any new changes. Using CVS, you don't
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have to download the entire source each time, only the changed files.
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Anonymous CVS does not allows developers to update the remote source
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tree, though privileged developers can do this. There is a CVS FAQ on
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our web site that describes how to use remote CVS. You can also use
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CVSup, which has similarly functionality, and is available from
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ftp.postgresql.org.
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To update the source tree, there are two ways. You can generate a
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patch against your current source tree, perhaps using the make_diff
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tools mentioned above, and send them to the patches list. They will be
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reviewed, and applied in a timely manner. If the patch is major, and
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we are in beta testing, the developers may wait for the final release
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before applying your patches.
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For hard-core developers, Marc(scrappy@postgresql.org) will give you a
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Unix shell account on postgresql.org, so you can use CVS to update the
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main source tree, or you can ftp your files into your account, patch,
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and cvs install the changes directly into the source tree.
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1.4) How do I test my changes?
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First, use psql to make sure it is working as you expect. Then run
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src/test/regress and get the output of src/test/regress/checkresults
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with and without your changes, to see that your patch does not change
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the regression test in unexpected ways. This practice has saved me
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many times. The regression tests test the code in ways I would never
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do, and has caught many bugs in my patches. By finding the problems
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now, you save yourself a lot of debugging later when things are
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broken, and you can't figure out when it happened.
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1.5) What tools are available for developers?
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Aside from the User documentation mentioned in the regular FAQ, there
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are several development tools available. First, all the files in the
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/tools directory are designed for developers.
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RELEASE_CHANGES changes we have to make for each release
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SQL_keywords standard SQL'92 keywords
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backend description/flowchart of the backend directories
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ccsym find standard defines made by your compiler
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entab converts tabs to spaces, used by pgindent
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find_static finds functions that could be made static
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find_typedef finds typedefs in the source code
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find_badmacros finds macros that use braces incorrectly
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make_ctags make vi 'tags' file in each directory
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make_diff make *.orig and diffs of source
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make_etags make emacs 'etags' files
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make_keywords make comparison of our keywords and SQL'92
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make_mkid make mkid ID files
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mkldexport create AIX exports file
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pgindent indents C source files
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pgjindent indents Java source files
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pginclude scripts for adding/removing include files
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unused_oids in pgsql/src/include/catalog
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Let me note some of these. If you point your browser at the
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file:/usr/local/src/pgsql/src/tools/backend/index.html directory, you
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will see few paragraphs describing the data flow, the backend
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components in a flow chart, and a description of the shared memory
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area. You can click on any flowchart box to see a description. If you
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then click on the directory name, you will be taken to the source
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directory, to browse the actual source code behind it. We also have
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several README files in some source directories to describe the
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function of the module. The browser will display these when you enter
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the directory also. The tools/backend directory is also contained on
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our web page under the title How PostgreSQL Processes a Query.
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Second, you really should have an editor that can handle tags, so you
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can tag a function call to see the function definition, and then tag
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inside that function to see an even lower-level function, and then
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back out twice to return to the original function. Most editors
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support this via tags or etags files.
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Third, you need to get id-utils from:
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ftp://alpha.gnu.org/gnu/id-utils-3.2d.tar.gz
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ftp://tug.org/gnu/id-utils-3.2d.tar.gz
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ftp://ftp.enst.fr/pub/gnu/gnits/id-utils-3.2d.tar.gz
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By running tools/make_mkid, an archive of source symbols can be
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created that can be rapidly queried like grep or edited. Others prefer
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glimpse.
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make_diff has tools to create patch diff files that can be applied to
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the distribution. This produces context diffs, which is our preferred
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format.
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Our standard format is to indent each code level with one tab, where
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each tab is four spaces. You will need to set your editor to display
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tabs as four spaces:
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vi in ~/.exrc:
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set tabstop=4
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set sw=4
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more:
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more -x4
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less:
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less -x4
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emacs:
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M-x set-variable tab-width
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or
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; Cmd to set tab stops & indenting for working with PostgreSQL code
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(c-add-style "pgsql"
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'("bsd"
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(indent-tabs-mode . t)
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(c-basic-offset . 4)
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(tab-width . 4)
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(c-offsets-alist .
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((case-label . +))))
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t) ; t = set this mode on
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and add this to your autoload list (modify file path in macro):
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(setq auto-mode-alist
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(cons '("\\`/usr/local/src/pgsql/.*\\.[chyl]\\'" . pgsql-c-mode)
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auto-mode-alist))
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or
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/*
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* Local variables:
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* tab-width: 4
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* c-indent-level: 4
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* c-basic-offset: 4
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* End:
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*/
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pgindent will the format code by specifying flags to your operating
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system's utility indent. This article describes the value of a
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constent coding style.
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pgindent is run on all source files just before each beta test period.
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It auto-formats all source files to make them consistent. Comment
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blocks that need specific line breaks should be formatted as block
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comments, where the comment starts as /*------. These comments will
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not be reformatted in any way.
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pginclude contains scripts used to add needed #include's to include
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files, and removed unneeded #include's.
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When adding system types, you will need to assign oids to them. There
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is also a script called unused_oids in pgsql/src/include/catalog that
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shows the unused oids.
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1.6) What books are good for developers?
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I have four good books, An Introduction to Database Systems, by C.J.
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Date, Addison, Wesley, A Guide to the SQL Standard, by C.J. Date, et.
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al, Addison, Wesley, Fundamentals of Database Systems, by Elmasri and
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Navathe, and Transaction Processing, by Jim Gray, Morgan, Kaufmann
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There is also a database performance site, with a handbook on-line
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written by Jim Gray at http://www.benchmarkresources.com.
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1.7) What is configure all about?
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The files configure and configure.in are part of the GNU autoconf
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package. Configure allows us to test for various capabilities of the
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OS, and to set variables that can then be tested in C programs and
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Makefiles. Autoconf is installed on the PostgreSQL main server. To add
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options to configure, edit configure.in, and then run autoconf to
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generate configure.
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When configure is run by the user, it tests various OS capabilities,
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stores those in config.status and config.cache, and modifies a list of
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*.in files. For example, if there exists a Makefile.in, configure
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generates a Makefile that contains substitutions for all @var@
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parameters found by configure.
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When you need to edit files, make sure you don't waste time modifying
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files generated by configure. Edit the *.in file, and re-run configure
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to recreate the needed file. If you run make distclean from the
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top-level source directory, all files derived by configure are
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removed, so you see only the file contained in the source
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distribution.
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1.8) How do I add a new port?
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There are a variety of places that need to be modified to add a new
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port. First, start in the src/template directory. Add an appropriate
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entry for your OS. Also, use src/config.guess to add your OS to
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src/template/.similar. You shouldn't match the OS version exactly. The
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configure test will look for an exact OS version number, and if not
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found, find a match without version number. Edit src/configure.in to
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add your new OS. (See configure item above.) You will need to run
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autoconf, or patch src/configure too.
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Then, check src/include/port and add your new OS file, with
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appropriate values. Hopefully, there is already locking code in
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src/include/storage/s_lock.h for your CPU. There is also a
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src/makefiles directory for port-specific Makefile handling. There is
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a backend/port directory if you need special files for your OS.
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1.9) Why don't we use threads in the backend?
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There are several reasons threads are not used:
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* Historically, threads were unsupported and buggy.
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* An error in one backend can corrupt other backends.
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* Speed improvements using threads are small compared to the
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remaining backend startup time.
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* The backend code would be more complex.
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1.10) How are RPM's packaged?
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This was written by Lamar Owen:
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2001-05-03
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As to how the RPMs are built -- to answer that question sanely
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requires me to know how much experience you have with the whole RPM
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paradigm. 'How is the RPM built?' is a multifaceted question. The
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obvious simple answer is that I maintain:
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1. A set of patches to make certain portions of the source tree
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'behave' in the different environment of the RPMset;
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2. The initscript;
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3. Any other ancilliary scripts and files;
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4. A README.rpm-dist document that tries to adequately document both
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the differences between the RPM build and the WHY of the
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differences, as well as useful RPM environment operations (like,
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using syslog, upgrading, getting postmaster to start at OS boot,
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etc);
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5. The spec file that throws it all together. This is not a trivial
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undertaking in a package of this size.
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I then download and build on as many different canonical distributions
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as I can -- currently I am able to build on Red Hat 6.2, 7.0, and 7.1
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on my personal hardware. Occasionally I receive opportunity from
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certain commercial enterprises such as Great Bridge and PostgreSQL,
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Inc. to build on other distributions.
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I test the build by installing the resulting packages and running the
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regression tests. Once the build passes these tests, I upload to the
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postgresql.org ftp server and make a release announcement. I am also
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responsible for maintaining the RPM download area on the ftp site.
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You'll notice I said 'canonical' distributions above. That simply
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means that the machine is as stock 'out of the box' as practical --
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that is, everything (except select few programs) on these boxen are
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installed by RPM; only official Red Hat released RPMs are used (except
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in unusual circumstances involving software that will not alter the
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build -- for example, installing a newer non-RedHat version of the Dia
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diagramming package is OK -- installing Python 2.1 on the box that has
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Python 1.5.2 installed is not, as that alters the PostgreSQL build).
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The RPM as uploaded is built to as close to out-of-the-box pristine as
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is possible. Only the standard released 'official to that release'
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compiler is used -- and only the standard official kernel is used as
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well.
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For a time I built on Mandrake for RedHat consumption -- no more.
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Nonstandard RPM building systems are worse than useless. Which is not
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to say that Mandrake is useless! By no means is Mandrake useless --
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unless you are building Red Hat RPMs -- and Red Hat is useless if
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you're trying to build Mandrake or SuSE RPMs, for that matter. But I
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would be foolish to use 'Lamar Owen's Super Special RPM Blend Distro
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0.1.2' to build for public consumption! :-)
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I _do_ attempt to make the _source_ RPM compatible with as many
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distributions as possible -- however, since I have limited resources
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(as a volunteer RPM maintainer) I am limited as to the amount of
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testing said build will get on other distributions, architectures, or
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systems.
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And, while I understand people's desire to immediately upgrade to the
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newest version, realize that I do this as a side interest -- I have a
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regular, full-time job as a broadcast
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engineer/webmaster/sysadmin/Technical Director which occasionally
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prevents me from making timely RPM releases. This happened during the
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early part of the 7.1 beta cycle -- but I believe I was pretty much on
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the ball for the Release Candidates and the final release.
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I am working towards a more open RPM distribution -- I would dearly
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love to more fully document the process and put everything into CVS --
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once I figure out how I want to represent things such as the spec file
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in a CVS form. It makes no sense to maintain a changelog, for
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instance, in the spec file in CVS when CVS does a better job of
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changelogs -- I will need to write a tool to generate a real spec file
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from a CVS spec-source file that would add version numbers, changelog
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entries, etc to the result before building the RPM. IOW, I need to
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rethink the process -- and then go through the motions of putting my
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long RPM history into CVS one version at a time so that version
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history information isn't lost.
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As to why all these files aren't part of the source tree, well, unless
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there was a large cry for it to happen, I don't believe it should.
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PostgreSQL is very platform-agnostic -- and I like that. Including the
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RPM stuff as part of the Official Tarball (TM) would, IMHO, slant that
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agnostic stance in a negative way. But maybe I'm too sensitive to
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that. I'm not opposed to doing that if that is the consensus of the
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core group -- and that would be a sneaky way to get the stuff into CVS
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:-). But if the core group isn't thrilled with the idea (and my
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instinct says they're not likely to be), I am opposed to the idea --
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not to keep the stuff to myself, but to not hinder the
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platform-neutral stance. IMHO, of course.
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Of course, there are many projects that DO include all the files
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necessary to build RPMs from their Official Tarball (TM).
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1.11) How are CVS branches managed?
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This was written by Tom Lane:
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2001-05-07
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If you just do basic "cvs checkout", "cvs update", "cvs commit", then
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you'll always be dealing with the HEAD version of the files in CVS.
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That's what you want for development, but if you need to patch past
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stable releases then you have to be able to access and update the
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"branch" portions of our CVS repository. We normally fork off a branch
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for a stable release just before starting the development cycle for
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the next release.
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The first thing you have to know is the branch name for the branch you
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are interested in getting at. To do this, look at some long-lived
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file, say the top-level HISTORY file, with "cvs status -v" to see what
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the branch names are. (Thanks to Ian Lance Taylor for pointing out
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that this is the easiest way to do it.) Typical branch names are:
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REL7_1_STABLE
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REL7_0_PATCHES
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REL6_5_PATCHES
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OK, so how do you do work on a branch? By far the best way is to
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create a separate checkout tree for the branch and do your work in
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that. Not only is that the easiest way to deal with CVS, but you
|
|
really need to have the whole past tree available anyway to test your
|
|
work. (And you *better* test your work. Never forget that dot-releases
|
|
tend to go out with very little beta testing --- so whenever you
|
|
commit an update to a stable branch, you'd better be doubly sure that
|
|
it's correct.)
|
|
|
|
Normally, to checkout the head branch, you just cd to the place you
|
|
want to contain the toplevel "pgsql" directory and say
|
|
cvs ... checkout pgsql
|
|
|
|
To get a past branch, you cd to whereever you want it and say
|
|
cvs ... checkout -r BRANCHNAME pgsql
|
|
|
|
For example, just a couple days ago I did
|
|
mkdir ~postgres/REL7_1
|
|
cd ~postgres/REL7_1
|
|
cvs ... checkout -r REL7_1_STABLE pgsql
|
|
|
|
and now I have a maintenance copy of 7.1.*.
|
|
|
|
When you've done a checkout in this way, the branch name is "sticky":
|
|
CVS automatically knows that this directory tree is for the branch,
|
|
and whenever you do "cvs update" or "cvs commit" in this tree, you'll
|
|
fetch or store the latest version in the branch, not the head version.
|
|
Easy as can be.
|
|
|
|
So, if you have a patch that needs to apply to both the head and a
|
|
recent stable branch, you have to make the edits and do the commit
|
|
twice, once in your development tree and once in your stable branch
|
|
tree. This is kind of a pain, which is why we don't normally fork the
|
|
tree right away after a major release --- we wait for a dot-release or
|
|
two, so that we won't have to double-patch the first wave of fixes.
|
|
|
|
1.12) Where can I get a copy of the SQL standards?
|
|
|
|
There are two pertinent standards, SQL92 and SQL99. These standards
|
|
are endorsed by ANSI and ISO. A draft of the SQL92 standard is
|
|
available at http://www.contrib.andrew.cmu.edu/~shadow/. The SQL99
|
|
standard must be purchased from ANSI at
|
|
http://webstore.ansi.org/ansidocstore/default.asp. The main standards
|
|
documents are ANSI X3.135-1992 for SQL92 and ANSI/ISO/IEC 9075-2-1999
|
|
for SQL99.
|
|
|
|
A summary of these standards is at
|
|
http://dbs.uni-leipzig.de/en/lokal/standards.pdf and
|
|
http://db.konkuk.ac.kr/present/SQL3.pdf.
|
|
|
|
Technical Questions
|
|
|
|
2.1) How do I efficiently access information in tables from the backend code?
|
|
|
|
You first need to find the tuples(rows) you are interested in. There
|
|
are two ways. First, SearchSysCache() and related functions allow you
|
|
to query the system catalogs. This is the preferred way to access
|
|
system tables, because the first call to the cache loads the needed
|
|
rows, and future requests can return the results without accessing the
|
|
base table. The caches use system table indexes to look up tuples. A
|
|
list of available caches is located in
|
|
src/backend/utils/cache/syscache.c.
|
|
src/backend/utils/cache/lsyscache.c contains many column-specific
|
|
cache lookup functions.
|
|
|
|
The rows returned are cache-owned versions of the heap rows.
|
|
Therefore, you must not modify or delete the tuple returned by
|
|
SearchSysCache(). What you should do is release it with
|
|
ReleaseSysCache() when you are done using it; this informs the cache
|
|
that it can discard that tuple if necessary. If you neglect to call
|
|
ReleaseSysCache(), then the cache entry will remain locked in the
|
|
cache until end of transaction, which is tolerable but not very
|
|
desirable.
|
|
|
|
If you can't use the system cache, you will need to retrieve the data
|
|
directly from the heap table, using the buffer cache that is shared by
|
|
all backends. The backend automatically takes care of loading the rows
|
|
into the buffer cache.
|
|
|
|
Open the table with heap_open(). You can then start a table scan with
|
|
heap_beginscan(), then use heap_getnext() and continue as long as
|
|
HeapTupleIsValid() returns true. Then do a heap_endscan(). Keys can be
|
|
assigned to the scan. No indexes are used, so all rows are going to be
|
|
compared to the keys, and only the valid rows returned.
|
|
|
|
You can also use heap_fetch() to fetch rows by block number/offset.
|
|
While scans automatically lock/unlock rows from the buffer cache, with
|
|
heap_fetch(), you must pass a Buffer pointer, and ReleaseBuffer() it
|
|
when completed.
|
|
|
|
Once you have the row, you can get data that is common to all tuples,
|
|
like t_self and t_oid, by merely accessing the HeapTuple structure
|
|
entries. If you need a table-specific column, you should take the
|
|
HeapTuple pointer, and use the GETSTRUCT() macro to access the
|
|
table-specific start of the tuple. You then cast the pointer as a
|
|
Form_pg_proc pointer if you are accessing the pg_proc table, or
|
|
Form_pg_type if you are accessing pg_type. You can then access the
|
|
columns by using a structure pointer:
|
|
((Form_pg_class) GETSTRUCT(tuple))->relnatts
|
|
|
|
You must not directly change live tuples in this way. The best way is
|
|
to use heap_modifytuple() and pass it your original tuple, and the
|
|
values you want changed. It returns a palloc'ed tuple, which you pass
|
|
to heap_replace(). You can delete tuples by passing the tuple's t_self
|
|
to heap_destroy(). You use t_self for heap_update() too. Remember,
|
|
tuples can be either system cache copies, which may go away after you
|
|
call ReleaseSysCache(), or read directly from disk buffers, which go
|
|
away when you heap_getnext(), heap_endscan, or ReleaseBuffer(), in the
|
|
heap_fetch() case. Or it may be a palloc'ed tuple, that you must
|
|
pfree() when finished.
|
|
|
|
2.2) Why are table, column, type, function, view names sometimes referenced
|
|
as Name or NameData, and sometimes as char *?
|
|
|
|
Table, column, type, function, and view names are stored in system
|
|
tables in columns of type Name. Name is a fixed-length,
|
|
null-terminated type of NAMEDATALEN bytes. (The default value for
|
|
NAMEDATALEN is 32 bytes.)
|
|
typedef struct nameData
|
|
{
|
|
char data[NAMEDATALEN];
|
|
} NameData;
|
|
typedef NameData *Name;
|
|
|
|
Table, column, type, function, and view names that come into the
|
|
backend via user queries are stored as variable-length,
|
|
null-terminated character strings.
|
|
|
|
Many functions are called with both types of names, ie. heap_open().
|
|
Because the Name type is null-terminated, it is safe to pass it to a
|
|
function expecting a char *. Because there are many cases where
|
|
on-disk names(Name) are compared to user-supplied names(char *), there
|
|
are many cases where Name and char * are used interchangeably.
|
|
|
|
2.3) Why do we use Node and List to make data structures?
|
|
|
|
We do this because this allows a consistent way to pass data inside
|
|
the backend in a flexible way. Every node has a NodeTag which
|
|
specifies what type of data is inside the Node. Lists are groups of
|
|
Nodes chained together as a forward-linked list.
|
|
|
|
Here are some of the List manipulation commands:
|
|
|
|
lfirst(i)
|
|
return the data at list element i.
|
|
|
|
lnext(i)
|
|
return the next list element after i.
|
|
|
|
foreach(i, list)
|
|
loop through list, assigning each list element to i. It is
|
|
important to note that i is a List *, not the data in the List
|
|
element. You need to use lfirst(i) to get at the data. Here is
|
|
a typical code snippet that loops through a List containing Var
|
|
*'s and processes each one:
|
|
|
|
List *i, *list;
|
|
|
|
foreach(i, list)
|
|
{
|
|
Var *var = lfirst(i);
|
|
|
|
/* process var here */
|
|
}
|
|
|
|
lcons(node, list)
|
|
add node to the front of list, or create a new list with node
|
|
if list is NIL.
|
|
|
|
lappend(list, node)
|
|
add node to the end of list. This is more expensive that lcons.
|
|
|
|
nconc(list1, list2)
|
|
Concat list2 on to the end of list1.
|
|
|
|
length(list)
|
|
return the length of the list.
|
|
|
|
nth(i, list)
|
|
return the i'th element in list.
|
|
|
|
lconsi, ...
|
|
There are integer versions of these: lconsi, lappendi, nthi.
|
|
List's containing integers instead of Node pointers are used to
|
|
hold list of relation object id's and other integer quantities.
|
|
|
|
You can print nodes easily inside gdb. First, to disable output
|
|
truncation when you use the gdb print command:
|
|
(gdb) set print elements 0
|
|
|
|
Instead of printing values in gdb format, you can use the next two
|
|
commands to print out List, Node, and structure contents in a verbose
|
|
format that is easier to understand. List's are unrolled into nodes,
|
|
and nodes are printed in detail. The first prints in a short format,
|
|
and the second in a long format:
|
|
(gdb) call print(any_pointer)
|
|
(gdb) call pprint(any_pointer)
|
|
|
|
The output appears in the postmaster log file, or on your screen if
|
|
you are running a backend directly without a postmaster.
|
|
|
|
2.4) I just added a field to a structure. What else should I do?
|
|
|
|
The structures passing around from the parser, rewrite, optimizer, and
|
|
executor require quite a bit of support. Most structures have support
|
|
routines in src/backend/nodes used to create, copy, read, and output
|
|
those structures. Make sure you add support for your new field to
|
|
these files. Find any other places the structure may need code for
|
|
your new field. mkid is helpful with this (see above).
|
|
|
|
2.5) Why do we use palloc() and pfree() to allocate memory?
|
|
|
|
palloc() and pfree() are used in place of malloc() and free() because
|
|
we find it easier to automatically free all memory allocated when a
|
|
query completes. This assures us that all memory that was allocated
|
|
gets freed even if we have lost track of where we allocated it. There
|
|
are special non-query contexts that memory can be allocated in. These
|
|
affect when the allocated memory is freed by the backend.
|
|
|
|
2.6) What is elog()?
|
|
|
|
elog() is used to send messages to the front-end, and optionally
|
|
terminate the current query being processed. The first parameter is an
|
|
elog level of DEBUG (levels 1-5), LOG, INFO, NOTICE, ERROR, FATAL, or
|
|
PANIC. NOTICE prints on the user's terminal and the postmaster logs.
|
|
INFO prints only to the user's terminal and LOG prints only to the
|
|
server logs. (These can be changed from postgresql.conf.) ERROR prints
|
|
in both places, and terminates the current query, never returning from
|
|
the call. FATAL terminates the backend process. The remaining
|
|
parameters of elog are a printf-style set of parameters to print.
|
|
|
|
elog(ERROR) frees most memory and open file descriptors so you don't
|
|
need to clean these up before the call.
|
|
|
|
2.7) What is CommandCounterIncrement()?
|
|
|
|
Normally, transactions can not see the rows they modify. This allows
|
|
UPDATE foo SET x = x + 1 to work correctly.
|
|
|
|
However, there are cases where a transactions needs to see rows
|
|
affected in previous parts of the transaction. This is accomplished
|
|
using a Command Counter. Incrementing the counter allows transactions
|
|
to be broken into pieces so each piece can see rows modified by
|
|
previous pieces. CommandCounterIncrement() increments the Command
|
|
Counter, creating a new part of the transaction.
|