postgresql/doc/FAQ_DEV

          Developer's Frequently Asked Questions (FAQ) for PostgreSQL
                                       
   Last updated: Wed Dec 1 16:11:11 EST 2006
   
   Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
   
   The most recent version of this document can be viewed at
   http://www.PostgreSQL.org/docs/faqs/FAQ_DEV.html.
     _________________________________________________________________
   
                             General Questions
                                      
   1.1) How do I get involved in PostgreSQL development?
   1.2) What development environment is required to develop code?
   1.3) What areas need work?
   1.4) What do I do after choosing an item to work on?
   1.5) Where can I learn more about the code?
   1.6) I've developed a patch, what next?
   1.7) How do I download/update the current source tree?
   1.8) How do I test my changes?
   1.9) What tools are available for developers?
   1.10) What books are good for developers?
   1.11) What is configure all about?
   1.12) How do I add a new port?
   1.13) Why don't you use threads/raw devices/async-I/O, <insert your
   favorite wizz-bang feature here>?
   1.14) How are RPM's packaged?
   1.15) How are CVS branches handled?
   1.16) Where can I get a copy of the SQL standards?
   1.17) Where can I get technical assistance?
   1.18) How do I get involved in PostgreSQL web site development?
   
                            Technical Questions
                                      
   2.1) How do I efficiently access information in tables from the
   backend code?
   2.2) Why are table, column, type, function, view names sometimes
   referenced as Name or NameData, and sometimes as char *?
   2.3) Why do we use Node and List to make data structures?
   2.4) I just added a field to a structure. What else should I do?
   2.5) Why do we use palloc() and pfree() to allocate memory?
   2.6) What is ereport()?
   2.7) What is CommandCounterIncrement()?
     _________________________________________________________________
   
                             General Questions
                                      
  1.1) How go I get involved in PostgreSQL development?
  
   Download the code and have a look around. See 1.7.
   
   Subscribe to and read the pgsql-hackers mailing list (often termed
   'hackers'). This is where the major contributors and core members of
   the project discuss development.
   
  1.2) What development environment is required to develop code?
  
   PostgreSQL is developed mostly in the C programming language. It also
   makes use of Yacc and Lex.
   
   The source code is targeted at most of the popular Unix platforms and
   the Windows environment (XP, Windows 2000, and up).
   
   Most developers make use of the open source development tool chain. If
   you have contributed to open source software before, you will probably
   be familiar with these tools. They include: GCC (http://gcc.gnu.org,
   GDB (www.gnu.org/software/gdb/gdb.html), autoconf
   (www.gnu.org/software/autoconf/) AND GNU make
   (www.gnu.org/software/make/make.html.
   
   Developers using this tool chain on Windows make use of MingW (see
   http://www.mingw.org/).
   
   Some developers use compilers from other software vendors with mixed
   results.
   
   Developers who are regularly rebuilding the source often pass the
   --enable-depend flag to configure. The result is that when you make a
   modification to a C header file, all files depend upon that file are
   also rebuilt.
   
  1.3) What areas need work?
  
   Outstanding features are detailed in the TODO list. This is located in
   doc/TODO in the source distribution or at
   http://developer.postgresql.org/todo.php.
   
   You can learn more about these features by consulting the archives,
   the SQL standards and the recommend texts (see 1.10).
   
  1.4) What do I do after choosing an item to work on?
  
   Send an email to pgsql-hackers with a proposal for what you want to do
   (assuming your contribution is not trivial). Working in isolation is
   not advisable: others may be working on the same TODO item; you may
   have misunderstood the TODO item; your approach may benefit from the
   review of others.
   
  1.5) Where can I learn more about the code?
  
   Other than documentation in the source tree itself, you can find some
   papers/presentations discussing the code at
   http://developers.postgresql.org.
   
  1.6) I've developed a patch, what next?
  
   Generate the patch in contextual diff format. If you are unfamiliar
   with this, you may find the script src/tools/makediff/difforig useful.
   
   Ensure that your patch is generated against the most recent version of
   the code. If it is a patch adding new functionality, the most recent
   version is cvs HEAD; if it is a bug fix, this will be the most
   recently version of the branch which suffers from the bug (for more on
   branches in PostgreSQL, see 1.15).
   
   Finally, submit the patch to pgsql-patches@postgresql.org. It will be
   reviewed by other contributors to the project and may be either
   accepted or sent back for further work.
   
  1.7) How do I download/update the current source tree?
  
   There are several ways to obtain the source tree. Occasional
   developers can just get the most recent source tree snapshot from
   ftp://ftp.postgresql.org.
   
   Regular developers may want to take advantage of anonymous access to
   our source code management system. The source tree is currently hosted
   in CVS. For details of how to obtain the source from CVS see
   http://developer.postgresql.org/docs/postgres/cvs.html.
   
  1.8) How do I test my changes?
  
   Basic system testing
   
   The easiest way to test your code is to ensure that it builds against
   the latest verion of the code and that it does not generate compiler
   warnings.
   
   It is worth advised that you pass --enable-cassert to configure. This
   will turn on assertions with in the source which will often show us
   bugs because they cause data corruption of segmentation violations.
   This generally makes debugging much easier.
   
   Then, perform run time testing via psql.
   
   Regression test suite
   
   The next step is to test your changes against the existing regression
   test suite. To do this, issue "make check" in the root directory of
   the source tree. If any tests failure, investigate.
   
   If you've deliberately changed existing behaviour, this change may
   cause a regression test failure but not any actual regression. If so,
   you should also patch the regression test suite.
   
   Other run time testing
   
   Some developers make use of tools such as valgrind
   (http://valgrind.kde.org) for memory testing, gprof (which comes with
   the GNU binutils suite) and oprofile
   (http://oprofile.sourceforge.net/) for profiling and other related
   tools.
   
   What about unit testing, static analysis, model checking...?
   
   There have been a number of discussions about other testing frameworks
   and some developers are exploring these ideas.
   
  1.9) What tools are available for developers?
  
   First, all the files in the src/tools directory are designed for
   developers.
    RELEASE_CHANGES changes we have to make for each release
    backend         description/flowchart of the backend directories
    ccsym           find standard defines made by your compiler
     copyright       fixes copyright notices

    entab           converts tabs to spaces, used by pgindent
    find_static     finds functions that could be made static
    find_typedef    finds typedefs in the source code
    find_badmacros  finds macros that use braces incorrectly
    fsync           a script to provide information about the cost of cache
                     syncing system calls
    make_ctags      make vi 'tags' file in each directory
    make_diff       make *.orig and diffs of source
    make_etags      make emacs 'etags' files
    make_keywords   make comparison of our keywords and SQL'92
    make_mkid       make mkid ID files
    pgcvslog        used to generate a list of changes for each release
    pginclude       scripts for adding/removing include files
    pgindent        indents source files
    pgtest          a semi-automated build system
    thread          a thread testing script

   In src/include/catalog:
    unused_oids     a script which generates unused OIDs for use in system
                     catalogs
    duplicate_oids  finds duplicate OIDs in system catalog definitions

   If you point your browser at the tools/backend/index.html file, you
   will see few paragraphs describing the data flow, the backend
   components in a flow chart, and a description of the shared memory
   area. You can click on any flowchart box to see a description. If you
   then click on the directory name, you will be taken to the source
   directory, to browse the actual source code behind it. We also have
   several README files in some source directories to describe the
   function of the module. The browser will display these when you enter
   the directory also. The tools/backend directory is also contained on
   our web page under the title How PostgreSQL Processes a Query.
   
   Second, you really should have an editor that can handle tags, so you
   can tag a function call to see the function definition, and then tag
   inside that function to see an even lower-level function, and then
   back out twice to return to the original function. Most editors
   support this via tags or etags files.
   
   Third, you need to get id-utils from ftp://ftp.gnu.org/gnu/id-utils/
   
   By running tools/make_mkid, an archive of source symbols can be
   created that can be rapidly queried.
   
   Some developers make use of cscope, which can be found at
   http://cscope.sf.net/. Others use glimpse, which can be found at
   http://webglimpse.net/.
   
   tools/make_diff has tools to create patch diff files that can be
   applied to the distribution. This produces context diffs, which is our
   preferred format.
   
   Our standard format is to indent each code level with one tab, where
   each tab is four spaces. You will need to set your editor to display
   tabs as four spaces:
    vi in ~/.exrc:
            set tabstop=4
            set sw=4
    more:
            more -x4
    less:
            less -x4
    emacs:
        M-x set-variable tab-width

        or

        (c-add-style "pgsql"
                '("bsd"
                        (indent-tabs-mode . t)
                        (c-basic-offset   . 4)
                        (tab-width . 4)
                        (c-offsets-alist .
                                ((case-label . +)))
                )
                nil ) ; t = set this style, nil = don't

        (defun pgsql-c-mode ()
                (c-mode)
                (c-set-style "pgsql")
        )

        and add this to your autoload list (modify file path in macro):

        (setq auto-mode-alist
                (cons '("\\`/home/andrew/pgsql/.*\\.[chyl]\\'" . pgsql-c-mode)
                auto-mode-alist))
        or
            /*
             * Local variables:
             *  tab-width: 4
             *  c-indent-level: 4
             *  c-basic-offset: 4
             * End:
             */

   pgindent will the format code by specifying flags to your operating
   system's utility indent. This article describes the value of a
   consistent coding style.
   
   pgindent is run on all source files just before each beta test period.
   It auto-formats all source files to make them consistent. Comment
   blocks that need specific line breaks should be formatted as block
   comments, where the comment starts as /*------. These comments will
   not be reformatted in any way.
   
   pginclude contains scripts used to add needed #include's to include
   files, and removed unneeded #include's.
   
   When adding system types, you will need to assign oids to them. There
   is also a script called unused_oids in pgsql/src/include/catalog that
   shows the unused oids.
   
  1.10) What books are good for developers?
  
   I have four good books, An Introduction to Database Systems, by C.J.
   Date, Addison, Wesley, A Guide to the SQL Standard, by C.J. Date, et.
   al, Addison, Wesley, Fundamentals of Database Systems, by Elmasri and
   Navathe, and Transaction Processing, by Jim Gray, Morgan, Kaufmann
   
   There is also a database performance site, with a handbook on-line
   written by Jim Gray at http://www.benchmarkresources.com..
   
  1.11) What is configure all about?
  
   The files configure and configure.in are part of the GNU autoconf
   package. Configure allows us to test for various capabilities of the
   OS, and to set variables that can then be tested in C programs and
   Makefiles. Autoconf is installed on the PostgreSQL main server. To add
   options to configure, edit configure.in, and then run autoconf to
   generate configure.
   
   When configure is run by the user, it tests various OS capabilities,
   stores those in config.status and config.cache, and modifies a list of
   *.in files. For example, if there exists a Makefile.in, configure
   generates a Makefile that contains substitutions for all @var@
   parameters found by configure.
   
   When you need to edit files, make sure you don't waste time modifying
   files generated by configure. Edit the *.in file, and re-run configure
   to recreate the needed file. If you run make distclean from the
   top-level source directory, all files derived by configure are
   removed, so you see only the file contained in the source
   distribution.
   
  1.12) How do I add a new port?
  
   There are a variety of places that need to be modified to add a new
   port. First, start in the src/template directory. Add an appropriate
   entry for your OS. Also, use src/config.guess to add your OS to
   src/template/.similar. You shouldn't match the OS version exactly. The
   configure test will look for an exact OS version number, and if not
   found, find a match without version number. Edit src/configure.in to
   add your new OS. (See configure item above.) You will need to run
   autoconf, or patch src/configure too.
   
   Then, check src/include/port and add your new OS file, with
   appropriate values. Hopefully, there is already locking code in
   src/include/storage/s_lock.h for your CPU. There is also a
   src/makefiles directory for port-specific Makefile handling. There is
   a backend/port directory if you need special files for your OS.
   
  1.13) Why don't you use threads/raw devices/async-I/O, <insert your favorite
  wizz-bang feature here>?
  
   There is always a temptation to use the newest operating system
   features as soon as they arrive. We resist that temptation.
   
   First, we support 15+ operating systems, so any new feature has to be
   well established before we will consider it. Second, most new
   wizz-bang features don't provide dramatic improvements. Third, they
   usually have some downside, such as decreased reliability or
   additional code required. Therefore, we don't rush to use new features
   but rather wait for the feature to be established, then ask for
   testing to show that a measurable improvement is possible.
   
   As an example, threads are not currently used in the backend code
   because:
     * Historically, threads were unsupported and buggy.
     * An error in one backend can corrupt other backends.
     * Speed improvements using threads are small compared to the
       remaining backend startup time.
     * The backend code would be more complex.
       
   So, we are not ignorant of new features. It is just that we are
   cautious about their adoption. The TODO list often contains links to
   discussions showing our reasoning in these areas.
   
  1.14) How are RPMs packaged?
  
   This was written by Lamar Owen:
   
   2001-05-03
   
   As to how the RPMs are built -- to answer that question sanely
   requires me to know how much experience you have with the whole RPM
   paradigm. 'How is the RPM built?' is a multifaceted question. The
   obvious simple answer is that I maintain:
    1. A set of patches to make certain portions of the source tree
       'behave' in the different environment of the RPMset;
    2. The initscript;
    3. Any other ancilliary scripts and files;
    4. A README.rpm-dist document that tries to adequately document both
       the differences between the RPM build and the WHY of the
       differences, as well as useful RPM environment operations (like,
       using syslog, upgrading, getting postmaster to start at OS boot,
       etc);
    5. The spec file that throws it all together. This is not a trivial
       undertaking in a package of this size.
       
   I then download and build on as many different canonical distributions
   as I can -- currently I am able to build on Red Hat 6.2, 7.0, and 7.1
   on my personal hardware. Occasionally I receive opportunity from
   certain commercial enterprises such as Great Bridge and PostgreSQL,
   Inc. to build on other distributions.
   
   I test the build by installing the resulting packages and running the
   regression tests. Once the build passes these tests, I upload to the
   postgresql.org ftp server and make a release announcement. I am also
   responsible for maintaining the RPM download area on the ftp site.
   
   You'll notice I said 'canonical' distributions above. That simply
   means that the machine is as stock 'out of the box' as practical --
   that is, everything (except select few programs) on these boxen are
   installed by RPM; only official Red Hat released RPMs are used (except
   in unusual circumstances involving software that will not alter the
   build -- for example, installing a newer non-RedHat version of the Dia
   diagramming package is OK -- installing Python 2.1 on the box that has
   Python 1.5.2 installed is not, as that alters the PostgreSQL build).
   The RPM as uploaded is built to as close to out-of-the-box pristine as
   is possible. Only the standard released 'official to that release'
   compiler is used -- and only the standard official kernel is used as
   well.
   
   For a time I built on Mandrake for RedHat consumption -- no more.
   Nonstandard RPM building systems are worse than useless. Which is not
   to say that Mandrake is useless! By no means is Mandrake useless --
   unless you are building Red Hat RPMs -- and Red Hat is useless if
   you're trying to build Mandrake or SuSE RPMs, for that matter. But I
   would be foolish to use 'Lamar Owen's Super Special RPM Blend Distro
   0.1.2' to build for public consumption! :-)
   
   I _do_ attempt to make the _source_ RPM compatible with as many
   distributions as possible -- however, since I have limited resources
   (as a volunteer RPM maintainer) I am limited as to the amount of
   testing said build will get on other distributions, architectures, or
   systems.
   
   And, while I understand people's desire to immediately upgrade to the
   newest version, realize that I do this as a side interest -- I have a
   regular, full-time job as a broadcast
   engineer/webmaster/sysadmin/Technical Director which occasionally
   prevents me from making timely RPM releases. This happened during the
   early part of the 7.1 beta cycle -- but I believe I was pretty much on
   the ball for the Release Candidates and the final release.
   
   I am working towards a more open RPM distribution -- I would dearly
   love to more fully document the process and put everything into CVS --
   once I figure out how I want to represent things such as the spec file
   in a CVS form. It makes no sense to maintain a changelog, for
   instance, in the spec file in CVS when CVS does a better job of
   changelogs -- I will need to write a tool to generate a real spec file
   from a CVS spec-source file that would add version numbers, changelog
   entries, etc to the result before building the RPM. IOW, I need to
   rethink the process -- and then go through the motions of putting my
   long RPM history into CVS one version at a time so that version
   history information isn't lost.
   
   As to why all these files aren't part of the source tree, well, unless
   there was a large cry for it to happen, I don't believe it should.
   PostgreSQL is very platform-agnostic -- and I like that. Including the
   RPM stuff as part of the Official Tarball (TM) would, IMHO, slant that
   agnostic stance in a negative way. But maybe I'm too sensitive to
   that. I'm not opposed to doing that if that is the consensus of the
   core group -- and that would be a sneaky way to get the stuff into CVS
   :-). But if the core group isn't thrilled with the idea (and my
   instinct says they're not likely to be), I am opposed to the idea --
   not to keep the stuff to myself, but to not hinder the
   platform-neutral stance. IMHO, of course.
   
   Of course, there are many projects that DO include all the files
   necessary to build RPMs from their Official Tarball (TM).
   
  1.15) How are CVS branches managed?
  
   This was written by Tom Lane:
   
   2001-05-07
   
   If you just do basic "cvs checkout", "cvs update", "cvs commit", then
   you'll always be dealing with the HEAD version of the files in CVS.
   That's what you want for development, but if you need to patch past
   stable releases then you have to be able to access and update the
   "branch" portions of our CVS repository. We normally fork off a branch
   for a stable release just before starting the development cycle for
   the next release.
   
   The first thing you have to know is the branch name for the branch you
   are interested in getting at. To do this, look at some long-lived
   file, say the top-level HISTORY file, with "cvs status -v" to see what
   the branch names are. (Thanks to Ian Lance Taylor for pointing out
   that this is the easiest way to do it.) Typical branch names are:
    REL7_1_STABLE
    REL7_0_PATCHES
    REL6_5_PATCHES

   OK, so how do you do work on a branch? By far the best way is to
   create a separate checkout tree for the branch and do your work in
   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.
   
  1.16) Where can I get a copy of the SQL standards?
  
   There are three versions of the SQL standard: SQL-92, SQL:1999, and
   SQL:2003. They are endorsed by ANSI and ISO. Draft versions can be
   downloaded from:
     * SQL-92 http://www.contrib.andrew.cmu.edu/~shadow/sql/sql1992.txt
     * SQL:1999
       http://www.cse.iitb.ac.in/dbms/Data/Papers-Other/SQL1999/ansi-iso-
       9075-2-1999.pdf
     * SQL:2003 http://www.wiscorp.com/sql/sql_2003_standard.zip
       
   Some SQL standards web pages are:
     * http://troels.arvin.dk/db/rdbms/links/#standards
     * http://www.wiscorp.com/SQLStandards.html
     * http://www.contrib.andrew.cmu.edu/~shadow/sql.html#syntax (SQL-92)
     * http://dbs.uni-leipzig.de/en/lokal/standards.pdf (paper)
       
  1.17) Where can I get technical assistance?
  
   Many technical questions held by those new to the code have been
   answered on the pgsql-hackers mailing list - the archives of which can
   be found at http://archives.postgresql.org/pgsql-hackers/.
   
   If you cannot find discussion or your particular question, feel free
   to put it to the list.
   
   Major contributors also answer technical questions, including
   questions about development of new features, on IRC at
   irc.freenode.net in the #postgresql channel.
   
  1.18) How go I get involved in PostgreSQL web site development?
  
   PostgreSQL website development is discussed on the
   pgsql-www@postgresql.org mailing list. The is a project page where the
   source code is available at
   http://gborg.postgresql.org/project/pgweb/projdisplay.php , the code
   for the next version of the website is under the "portal" module. You
   will al so find code for the "techdocs" website if you would like to
   contribute to that. A temporary todo list for current website
   development issues is available at http://xzilla.postgresql.org/todo
   
                            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 64 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), lfirst_int(i), lfirst_oid(i)
          return the data (a point, inteter and OID respectively) 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                *list;
    ListCell    *i;

    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, etc.
          Also versions for OID lists: lconso, lappendo, etc.
          
   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 (in particular, the files copyfuncs.c and
   equalfuncs.c. 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 1.9).
   
  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 ereport()?
  
   ereport() is used to send messages to the front-end, and optionally
   terminate the current query being processed. The first parameter is an
   ereport 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 ereport are a printf-style set of parameters
   to print.
   
   ereport(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.