From 47309464e4e937e1b11320eab9b0eff9ad63cd80 Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Fri, 31 Oct 2003 22:41:21 +0000 Subject: [PATCH] Rewrite GiST documentation into something actually useful. Christopher Kings-Lynne --- doc/src/sgml/gist.sgml | 352 +++++++++++++++++++++++++++++------------ 1 file changed, 251 insertions(+), 101 deletions(-) diff --git a/doc/src/sgml/gist.sgml b/doc/src/sgml/gist.sgml index 4354d8a4b6..6b34e498ea 100644 --- a/doc/src/sgml/gist.sgml +++ b/doc/src/sgml/gist.sgml @@ -1,113 +1,263 @@ - - - - -Gene -Selkov - - -Transcribed 1998-02-19 - -GiST Indexes + +GiST Indexes - -The information about GIST is at - http://GiST.CS.Berkeley.EDU:8000/gist/ + + Introduction -with more on different indexing and sorting schemes at -http://s2k-ftp.CS.Berkeley.EDU:8000/personal/jmh/. + + GiST stands for Generalized Search Tree. It is a + balanced, tree-structured access method, that acts as a base template in + which to implement arbitrary indexing schemes. B+-trees, R-trees and many + other indexing schemes can be implemented in GiST. + -And there is more interesting reading at -http://epoch.cs.berkeley.edu:8000/ and -http://www.sai.msu.su/~megera/postgres/gist/. - + + One advantage of GiST is that it allows the development + of custom data types with the appropriate access methods, by + an expert in the domain of the data type, rather than a database expert. + - - -Author - -This extraction from an email sent by -Eugene Selkov, Jr. (selkovjr@mcs.anl.gov) -contains good information -on GiST. Hopefully we will learn more in the future and update this information. -- thomas 1998-03-01 - - - - -Well, I can't say I quite understand what's going on, but at least -I (almost) succeeded in porting GiST examples to linux. The GiST access -method is already in the postgres tree (src/backend/access/gist). - - -Examples at Berkeley -come with an overview of the methods and demonstrate spatial index -mechanisms for 2D boxes, polygons, integer intervals and text -(see also GiST at Berkeley). -In the box example, we -are supposed to see a performance gain when using the GiST index; it did -work for me but I do not have a reasonably large collection of boxes -to check that. Other examples also worked, except polygons: I got an -error doing + + Some of the information here is derived from the University of California at + Berkeley's GiST Indexing Project web site and Marcel Kornacker's + thesis, + Access Methods for + Next-Generation Database Systems. The GiST + implementation in PostgreSQL is primarily + maintained by Teodor Sigaev and Oleg Bartunov, and there is more + information on their website: . + - -test=> CREATE INDEX pix ON polytmp -test-> USING GIST (p:box gist_poly_ops) WITH (ISLOSSY); -ERROR: cannot open pix + -(PostgreSQL 6.3 Sun Feb 1 14:57:30 EST 1998) - - - -I could not get sense of this error message; it appears to be something -we'd rather ask the developers about (see also Note 4 below). What I -would suggest here is that someone of you linux guys (linux==gcc?) fetch the -original sources quoted above and apply my patch (see attachment) and -tell us what you feel about it. Looks cool to me, but I would not like -to hold it up while there are so many competent people around. - - -A few notes on the sources: - - -1. I failed to make use of the original (HP-UX) Makefile and rearranged - the Makefile from the ancient postgres95 tutorial to do the job. I tried - to keep it generic, but I am a very poor makefile writer -- just did - some monkey work. Sorry about that, but I guess it is now a little - more portable that the original makefile. - - -2. I built the example sources right under pgsql/src (just extracted the - tar file there). The aforementioned Makefile assumes it is one level - below pgsql/src (in our case, in pgsql/src/pggist). - - -3. The changes I made to the *.c files were all about #include's, - function prototypes and typecasting. Other than that, I just threw - away a bunch of unused vars and added a couple parentheses to please - gcc. I hope I did not screw up too much :) - - -4. There is a comment in polyproc.sql: + + Extensibility - --- -- there's a memory leak in rtree poly_ops!! --- -- CREATE INDEX pix2 ON polytmp USING RTREE (p poly_ops); - + + Traditionally, implementing a new index access method meant a lot of + difficult work. It was necessary to understand the inner workings of the + database, such as the lock manager and Write-Ahead Log. The + GiST interface has a high level of abstraction, + requiring the access method implementor to only implement the semantics of + the data type being accessed. The GiST layer itself + takes care of concurrency, logging and searching the tree structure. + + + + This extensibility should not be confused with the extensibility of the + other standard search trees in terms of the data they can handle. For + example, PostgreSQL supports extensible B+-trees + and R-trees. That means that you can use + PostgreSQL to build a B+-tree or R-tree over any + data type you want. But B+-trees only support range predicates + (<, =, >), + and R-trees only support n-D range queries (contains, contained, equals). + + + + So if you index, say, an image collection with a + PostgreSQL B+-tree, you can only issue queries + such as is imagex equal to imagey, is imagex less + than imagey and is imagex greater than imagey? + Depending on how you define equals, less than + and greater than in this context, this could be useful. + However, by using a GiST based index, you could create + ways to ask domain-specific questions, perhaps find all images of + horses or find all over-exposed images. + - Roger that!! I thought it could be related to a number of - PostgreSQL versions - back and tried the query. My system went nuts and I had to shoot down - the postmaster in about ten minutes. - + + All it takes to get a GiST access method up and running + is to implement seven user-defined methods, which define the behavior of + keys in the tree. Of course these methods have to be pretty fancy to + support fancy queries, but for all the standard queries (B+-trees, + R-trees, etc.) they're relatively straightforward. In short, + GiST combines extensibility along with generality, code + reuse, and a clean interface. + - -I will continue to look into GiST for a while, but I would also -appreciate -more examples of R-tree usage. - - + + + + Implementation + + + There are seven methods that an index operator class for + GiST must provide: + + + + + consistent + + + Given a predicate p on a tree page, and a user + query, q, this method will return false if it is + certain that both p and q cannot + be true for a given data item. + + + + + + union + + + This method consolidates information in the tree. Given a set of + entries, this function generates a new predicate that is true for all + the entries. + + + + + + compress + + + Converts the data item into a format suitable for physical storage in + an index page. + + + + + + decompress + + + The reverse of the compress method. Converts the + index representation of the data item into a format that can be + manipulated by the database. + + + + + + penalty + + + Returns a value indicating the cost of inserting the new + entry into a particular branch of the tree. items will be inserted + down the path of least penalty in the tree. + + + + + + picksplit + + + When a page split is necessary, this function decides which entries on + the page are to stay on the old page, and which are to move to the new + page. + + + + + + same + + + Returns true if two entries are identical, false otherwise. + + + + + + + + + + Limitations + + + The current implementation of GiST within + PostgreSQL has some major limitations: + GiST access is not concurrent; the + GiST interface doesn't allow the development of certain + data types, such as digital trees (see papers by Aoki et al); and there + is not yet any support for write-ahead logging of updates in + GiST indexes. + + + + Solutions to the concurrency problems appear in Marcel Kornacker's + thesis; however these ideas have not yet been put into practice in the + PostgreSQL implementation. + + + + The lack of write-ahead logging is just a small matter of programming, + but since it isn't done yet, a crash could render a GiST + index inconsistent, forcing a REINDEX. + + + + + + Examples + + + To see example implementations of index methods implemented using + GiST, examine the following contrib modules: + + + + + btree_gist + + B-Tree + + + + + cube + + Indexing for multi-dimensional cubes + + + + + intarray + + RD-Tree for one-dimensional array of int4 values + + + + + ltree + + Indexing for tree-like stuctures + + + + + rtree_gist + + R-Tree + + + + + seg + + Storage and indexed access for float ranges + + + + + tsearch and tsearch2 + + Full text indexing + + + + + + +