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<!-- doc/src/sgml/gist.sgml -->
<chapter id="gist">
<title>GiST Indexes</title>
<indexterm>
<primary>index</primary>
<secondary>GiST</secondary>
</indexterm>
<sect1 id="gist-intro">
<title>Introduction</title>
<para>
<acronym>GiST</acronym> 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 <acronym>GiST</acronym>.
</para>
<para>
One advantage of <acronym>GiST</acronym> 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.
</para>
<para>
Some of the information here is derived from the University of California
at Berkeley's GiST Indexing Project
<ulink url="http://gist.cs.berkeley.edu/">web site</ulink> and
Marcel Kornacker's thesis,
<ulink url="http://www.sai.msu.su/~megera/postgres/gist/papers/concurrency/access-methods-for-next-generation.pdf.gz">
Access Methods for Next-Generation Database Systems</ulink>.
The <acronym>GiST</acronym>
implementation in <productname>PostgreSQL</productname> is primarily
maintained by Teodor Sigaev and Oleg Bartunov, and there is more
information on their
<ulink url="http://www.sai.msu.su/~megera/postgres/gist/">web site</ulink>.
</para>
</sect1>
<sect1 id="gist-builtin-opclasses">
<title>Built-in Operator Classes</title>
<para>
The core <productname>PostgreSQL</productname> distribution
includes the <acronym>GiST</acronym> operator classes shown in
<xref linkend="gist-builtin-opclasses-table"/>.
(Some of the optional modules described in <xref linkend="contrib"/>
provide additional <acronym>GiST</acronym> operator classes.)
</para>
<table id="gist-builtin-opclasses-table">
<title>Built-in <acronym>GiST</acronym> Operator Classes</title>
<tgroup cols="3">
<colspec colname="col1" colwidth="2*"/>
<colspec colname="col2" colwidth="3*"/>
<colspec colname="col3" colwidth="2*"/>
<thead>
<row>
<entry>Name</entry>
<entry>Indexable Operators</entry>
<entry>Ordering Operators</entry>
</row>
</thead>
<tbody>
<row>
<entry valign="middle" morerows="13"><literal>box_ops</literal></entry>
<entry><literal><< (box, box)</literal></entry>
<entry valign="middle" morerows="13"><literal><-> (box, point)</literal></entry>
</row>
<row><entry><literal>&< (box, box)</literal></entry></row>
<row><entry><literal>&& (box, box)</literal></entry></row>
<row><entry><literal>&> (box, box)</literal></entry></row>
<row><entry><literal>>> (box, box)</literal></entry></row>
<row><entry><literal>~= (box, box)</literal></entry></row>
<row><entry><literal>@> (box, box)</literal></entry></row>
<row><entry><literal><@ (box, box)</literal></entry></row>
<row><entry><literal>&<| (box, box)</literal></entry></row>
<row><entry><literal><<| (box, box)</literal></entry></row>
<row><entry><literal>|>> (box, box)</literal></entry></row>
<row><entry><literal>|&> (box, box)</literal></entry></row>
<row><entry><literal>~ (box, box)</literal></entry></row>
<row><entry><literal>@ (box, box)</literal></entry></row>
<row>
<entry valign="middle" morerows="13"><literal>circle_ops</literal></entry>
<entry><literal><< (circle, circle)</literal></entry>
<entry valign="middle" morerows="13"><literal><-> (circle, point)</literal></entry>
</row>
<row><entry><literal>&< (circle, circle)</literal></entry></row>
<row><entry><literal>&> (circle, circle)</literal></entry></row>
<row><entry><literal>>> (circle, circle)</literal></entry></row>
<row><entry><literal><@ (circle, circle)</literal></entry></row>
<row><entry><literal>@> (circle, circle)</literal></entry></row>
<row><entry><literal>~= (circle, circle)</literal></entry></row>
<row><entry><literal>&& (circle, circle)</literal></entry></row>
<row><entry><literal>|>> (circle, circle)</literal></entry></row>
<row><entry><literal><<| (circle, circle)</literal></entry></row>
<row><entry><literal>&<| (circle, circle)</literal></entry></row>
<row><entry><literal>|&> (circle, circle)</literal></entry></row>
<row><entry><literal>@ (circle, circle)</literal></entry></row>
<row><entry><literal>~ (circle, circle)</literal></entry></row>
<row>
<entry valign="middle" morerows="10"><literal>inet_ops</literal></entry>
<entry><literal><< (inet, inet)</literal></entry>
<entry valign="middle" morerows="10"></entry>
</row>
<row><entry><literal><<= (inet, inet)</literal></entry></row>
<row><entry><literal>>> (inet, inet)</literal></entry></row>
<row><entry><literal>>>= (inet, inet)</literal></entry></row>
<row><entry><literal>= (inet, inet)</literal></entry></row>
<row><entry><literal><> (inet, inet)</literal></entry></row>
<row><entry><literal>< (inet, inet)</literal></entry></row>
<row><entry><literal><= (inet, inet)</literal></entry></row>
<row><entry><literal>> (inet, inet)</literal></entry></row>
<row><entry><literal>>= (inet, inet)</literal></entry></row>
<row><entry><literal>&& (inet, inet)</literal></entry></row>
<row>
<entry valign="middle" morerows="17"><literal>multirange_ops</literal></entry>
<entry><literal>= (anymultirange, anymultirange)</literal></entry>
<entry valign="middle" morerows="17"></entry>
</row>
<row><entry><literal>&& (anymultirange, anymultirange)</literal></entry></row>
<row><entry><literal>&& (anymultirange, anyrange)</literal></entry></row>
<row><entry><literal>@> (anymultirange, anyelement)</literal></entry></row>
<row><entry><literal>@> (anymultirange, anymultirange)</literal></entry></row>
<row><entry><literal>@> (anymultirange, anyrange)</literal></entry></row>
<row><entry><literal><@ (anymultirange, anymultirange)</literal></entry></row>
<row><entry><literal><@ (anymultirange, anyrange)</literal></entry></row>
<row><entry><literal><< (anymultirange, anymultirange)</literal></entry></row>
<row><entry><literal><< (anymultirange, anyrange)</literal></entry></row>
<row><entry><literal>>> (anymultirange, anymultirange)</literal></entry></row>
<row><entry><literal>>> (anymultirange, anyrange)</literal></entry></row>
<row><entry><literal>&< (anymultirange, anymultirange)</literal></entry></row>
<row><entry><literal>&< (anymultirange, anyrange)</literal></entry></row>
<row><entry><literal>&> (anymultirange, anymultirange)</literal></entry></row>
<row><entry><literal>&> (anymultirange, anyrange)</literal></entry></row>
<row><entry><literal>-|- (anymultirange, anymultirange)</literal></entry></row>
<row><entry><literal>-|- (anymultirange, anyrange)</literal></entry></row>
<row>
<entry valign="middle" morerows="7"><literal>point_ops</literal></entry>
<entry><literal>|>> (point, point)</literal></entry>
<entry valign="middle" morerows="7"><literal><-> (point, point)</literal></entry>
</row>
<row><entry><literal><< (point, point)</literal></entry></row>
<row><entry><literal>>> (point, point)</literal></entry></row>
<row><entry><literal><<| (point, point)</literal></entry></row>
<row><entry><literal>~= (point, point)</literal></entry></row>
<row><entry><literal><@ (point, box)</literal></entry></row>
<row><entry><literal><@ (point, polygon)</literal></entry></row>
<row><entry><literal><@ (point, circle)</literal></entry></row>
<row>
<entry valign="middle" morerows="13"><literal>poly_ops</literal></entry>
<entry><literal><< (polygon, polygon)</literal></entry>
<entry valign="middle" morerows="13"><literal><-> (polygon, point)</literal></entry>
</row>
<row><entry><literal>&< (polygon, polygon)</literal></entry></row>
<row><entry><literal>&> (polygon, polygon)</literal></entry></row>
<row><entry><literal>>> (polygon, polygon)</literal></entry></row>
<row><entry><literal><@ (polygon, polygon)</literal></entry></row>
<row><entry><literal>@> (polygon, polygon)</literal></entry></row>
<row><entry><literal>~= (polygon, polygon)</literal></entry></row>
<row><entry><literal>&& (polygon, polygon)</literal></entry></row>
<row><entry><literal><<| (polygon, polygon)</literal></entry></row>
<row><entry><literal>&<| (polygon, polygon)</literal></entry></row>
<row><entry><literal>|&> (polygon, polygon)</literal></entry></row>
<row><entry><literal>|>> (polygon, polygon)</literal></entry></row>
<row><entry><literal>@ (polygon, polygon)</literal></entry></row>
<row><entry><literal>~ (polygon, polygon)</literal></entry></row>
<row>
<entry valign="middle" morerows="17"><literal>range_ops</literal></entry>
<entry><literal>= (anyrange, anyrange)</literal></entry>
<entry valign="middle" morerows="17"></entry>
</row>
<row><entry><literal>&& (anyrange, anyrange)</literal></entry></row>
<row><entry><literal>&& (anyrange, anymultirange)</literal></entry></row>
<row><entry><literal>@> (anyrange, anyelement)</literal></entry></row>
<row><entry><literal>@> (anyrange, anyrange)</literal></entry></row>
<row><entry><literal>@> (anyrange, anymultirange)</literal></entry></row>
<row><entry><literal><@ (anyrange, anyrange)</literal></entry></row>
<row><entry><literal><@ (anyrange, anymultirange)</literal></entry></row>
<row><entry><literal><< (anyrange, anyrange)</literal></entry></row>
<row><entry><literal><< (anyrange, anymultirange)</literal></entry></row>
<row><entry><literal>>> (anyrange, anyrange)</literal></entry></row>
<row><entry><literal>>> (anyrange, anymultirange)</literal></entry></row>
<row><entry><literal>&< (anyrange, anyrange)</literal></entry></row>
<row><entry><literal>&< (anyrange, anymultirange)</literal></entry></row>
<row><entry><literal>&> (anyrange, anyrange)</literal></entry></row>
<row><entry><literal>&> (anyrange, anymultirange)</literal></entry></row>
<row><entry><literal>-|- (anyrange, anyrange)</literal></entry></row>
<row><entry><literal>-|- (anyrange, anymultirange)</literal></entry></row>
<row>
<entry valign="middle" morerows="1"><literal>tsquery_ops</literal></entry>
<entry><literal><@ (tsquery, tsquery)</literal></entry>
<entry valign="middle" morerows="1"></entry>
</row>
<row><entry><literal>@> (tsquery, tsquery)</literal></entry></row>
<row>
<entry valign="middle"><literal>tsvector_ops</literal></entry>
<entry><literal>@@ (tsvector, tsquery)</literal></entry>
<entry></entry>
</row>
</tbody>
</tgroup>
</table>
<para>
For historical reasons, the <literal>inet_ops</literal> operator class is
not the default class for types <type>inet</type> and <type>cidr</type>.
To use it, mention the class name in <command>CREATE INDEX</command>,
for example
<programlisting>
CREATE INDEX ON my_table USING GIST (my_inet_column inet_ops);
</programlisting>
</para>
</sect1>
<sect1 id="gist-extensibility">
<title>Extensibility</title>
<para>
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
<acronym>GiST</acronym> interface has a high level of abstraction,
requiring the access method implementer only to implement the semantics of
the data type being accessed. The <acronym>GiST</acronym> layer itself
takes care of concurrency, logging and searching the tree structure.
</para>
<para>
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, <productname>PostgreSQL</productname> supports extensible B-trees
and hash indexes. That means that you can use
<productname>PostgreSQL</productname> to build a B-tree or hash over any
data type you want. But B-trees only support range predicates
(<literal><</literal>, <literal>=</literal>, <literal>></literal>),
and hash indexes only support equality queries.
</para>
<para>
So if you index, say, an image collection with a
<productname>PostgreSQL</productname> B-tree, you can only issue queries
such as <quote>is imagex equal to imagey</quote>, <quote>is imagex less
than imagey</quote> and <quote>is imagex greater than imagey</quote>.
Depending on how you define <quote>equals</quote>, <quote>less than</quote>
and <quote>greater than</quote> in this context, this could be useful.
However, by using a <acronym>GiST</acronym> based index, you could create
ways to ask domain-specific questions, perhaps <quote>find all images of
horses</quote> or <quote>find all over-exposed images</quote>.
</para>
<para>
All it takes to get a <acronym>GiST</acronym> access method up and running
is to implement several 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,
<acronym>GiST</acronym> combines extensibility along with generality, code
reuse, and a clean interface.
</para>
<para>
There are five methods that an index operator class for
<acronym>GiST</acronym> must provide, and seven that are optional.
Correctness of the index is ensured
by proper implementation of the <function>same</function>, <function>consistent</function>
and <function>union</function> methods, while efficiency (size and speed) of the
index will depend on the <function>penalty</function> and <function>picksplit</function>
methods.
Two optional methods are <function>compress</function> and
<function>decompress</function>, which allow an index to have internal tree data of
a different type than the data it indexes. The leaves are to be of the
indexed data type, while the other tree nodes can be of any C struct (but
you still have to follow <productname>PostgreSQL</productname> data type rules here,
see about <literal>varlena</literal> for variable sized data). If the tree's
internal data type exists at the SQL level, the <literal>STORAGE</literal> option
of the <command>CREATE OPERATOR CLASS</command> command can be used.
The optional eighth method is <function>distance</function>, which is needed
if the operator class wishes to support ordered scans (nearest-neighbor
searches). The optional ninth method <function>fetch</function> is needed if the
operator class wishes to support index-only scans, except when the
<function>compress</function> method is omitted. The optional tenth method
<function>options</function> is needed if the operator class has
user-specified parameters.
The optional eleventh method <function>sortsupport</function> is used to
speed up building a <acronym>GiST</acronym> index.
The optional twelfth method <function>stratnum</function> is used to
translate well-known <literal>RT*StrategyNumber</literal>s (from
<filename>src/include/access/stratnum.h</filename>) into strategy numbers
used by the operator class. This lets the core code look up operators for
temporal constraint indexes.
</para>
<variablelist>
<varlistentry>
<term><function>consistent</function></term>
<listitem>
<para>
Given an index entry <literal>p</literal> and a query value <literal>q</literal>,
this function determines whether the index entry is
<quote>consistent</quote> with the query; that is, could the predicate
<quote><replaceable>indexed_column</replaceable>
<replaceable>indexable_operator</replaceable> <literal>q</literal></quote> be true for
any row represented by the index entry? For a leaf index entry this is
equivalent to testing the indexable condition, while for an internal
tree node this determines whether it is necessary to scan the subtree
of the index represented by the tree node. When the result is
<literal>true</literal>, a <literal>recheck</literal> flag must also be returned.
This indicates whether the predicate is certainly true or only possibly
true. If <literal>recheck</literal> = <literal>false</literal> then the index has
tested the predicate condition exactly, whereas if <literal>recheck</literal>
= <literal>true</literal> the row is only a candidate match. In that case the
system will automatically evaluate the
<replaceable>indexable_operator</replaceable> against the actual row value to see
if it is really a match. This convention allows
<acronym>GiST</acronym> to support both lossless and lossy index
structures.
</para>
<para>
The <acronym>SQL</acronym> declaration of the function must look like this:
<programlisting>
CREATE OR REPLACE FUNCTION my_consistent(internal, data_type, smallint, oid, internal)
RETURNS bool
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;
</programlisting>
And the matching code in the C module could then follow this skeleton:
<programlisting>
PG_FUNCTION_INFO_V1(my_consistent);
Datum
my_consistent(PG_FUNCTION_ARGS)
{
GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
data_type *query = PG_GETARG_DATA_TYPE_P(1);
StrategyNumber strategy = (StrategyNumber) PG_GETARG_UINT16(2);
/* Oid subtype = PG_GETARG_OID(3); */
bool *recheck = (bool *) PG_GETARG_POINTER(4);
data_type *key = DatumGetDataType(entry->key);
bool retval;
/*
* determine return value as a function of strategy, key and query.
*
* Use GIST_LEAF(entry) to know where you're called in the index tree,
* which comes handy when supporting the = operator for example (you could
* check for non empty union() in non-leaf nodes and equality in leaf
* nodes).
*/
*recheck = true; /* or false if check is exact */
PG_RETURN_BOOL(retval);
}
</programlisting>
Here, <varname>key</varname> is an element in the index and <varname>query</varname>
the value being looked up in the index. The <literal>StrategyNumber</literal>
parameter indicates which operator of your operator class is being
applied — it matches one of the operator numbers in the
<command>CREATE OPERATOR CLASS</command> command.
</para>
<para>
Depending on which operators you have included in the class, the data
type of <varname>query</varname> could vary with the operator, since it will
be whatever type is on the right-hand side of the operator, which might
be different from the indexed data type appearing on the left-hand side.
(The above code skeleton assumes that only one type is possible; if
not, fetching the <varname>query</varname> argument value would have to depend
on the operator.) It is recommended that the SQL declaration of
the <function>consistent</function> function use the opclass's indexed data
type for the <varname>query</varname> argument, even though the actual type
might be something else depending on the operator.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>union</function></term>
<listitem>
<para>
This method consolidates information in the tree. Given a set of
entries, this function generates a new index entry that represents
all the given entries.
</para>
<para>
The <acronym>SQL</acronym> declaration of the function must look like this:
<programlisting>
CREATE OR REPLACE FUNCTION my_union(internal, internal)
RETURNS storage_type
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;
</programlisting>
And the matching code in the C module could then follow this skeleton:
<programlisting>
PG_FUNCTION_INFO_V1(my_union);
Datum
my_union(PG_FUNCTION_ARGS)
{
GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0);
GISTENTRY *ent = entryvec->vector;
data_type *out,
*tmp,
*old;
int numranges,
i = 0;
numranges = entryvec->n;
tmp = DatumGetDataType(ent[0].key);
out = tmp;
if (numranges == 1)
{
out = data_type_deep_copy(tmp);
PG_RETURN_DATA_TYPE_P(out);
}
for (i = 1; i < numranges; i++)
{
old = out;
tmp = DatumGetDataType(ent[i].key);
out = my_union_implementation(out, tmp);
}
PG_RETURN_DATA_TYPE_P(out);
}
</programlisting>
</para>
<para>
As you can see, in this skeleton we're dealing with a data type
where <literal>union(X, Y, Z) = union(union(X, Y), Z)</literal>. It's easy
enough to support data types where this is not the case, by
implementing the proper union algorithm in this
<acronym>GiST</acronym> support method.
</para>
<para>
The result of the <function>union</function> function must be a value of the
index's storage type, whatever that is (it might or might not be
different from the indexed column's type). The <function>union</function>
function should return a pointer to newly <function>palloc()</function>ed
memory. You can't just return the input value as-is, even if there is
no type change.
</para>
<para>
As shown above, the <function>union</function> function's
first <type>internal</type> argument is actually
a <structname>GistEntryVector</structname> pointer. The second argument is a
pointer to an integer variable, which can be ignored. (It used to be
required that the <function>union</function> function store the size of its
result value into that variable, but this is no longer necessary.)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>compress</function></term>
<listitem>
<para>
Converts a data item into a format suitable for physical storage in
an index page.
If the <function>compress</function> method is omitted, data items are stored
in the index without modification.
</para>
<para>
The <acronym>SQL</acronym> declaration of the function must look like this:
<programlisting>
CREATE OR REPLACE FUNCTION my_compress(internal)
RETURNS internal
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;
</programlisting>
And the matching code in the C module could then follow this skeleton:
<programlisting>
PG_FUNCTION_INFO_V1(my_compress);
Datum
my_compress(PG_FUNCTION_ARGS)
{
GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
GISTENTRY *retval;
if (entry->leafkey)
{
/* replace entry->key with a compressed version */
compressed_data_type *compressed_data = palloc(sizeof(compressed_data_type));
/* fill *compressed_data from entry->key ... */
retval = palloc(sizeof(GISTENTRY));
gistentryinit(*retval, PointerGetDatum(compressed_data),
entry->rel, entry->page, entry->offset, FALSE);
}
else
{
/* typically we needn't do anything with non-leaf entries */
retval = entry;
}
PG_RETURN_POINTER(retval);
}
</programlisting>
</para>
<para>
You have to adapt <replaceable>compressed_data_type</replaceable> to the specific
type you're converting to in order to compress your leaf nodes, of
course.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>decompress</function></term>
<listitem>
<para>
Converts the stored representation of a data item into a format that
can be manipulated by the other GiST methods in the operator class.
If the <function>decompress</function> method is omitted, it is assumed that
the other GiST methods can work directly on the stored data format.
(<function>decompress</function> is not necessarily the reverse of
the <function>compress</function> method; in particular,
if <function>compress</function> is lossy then it's impossible
for <function>decompress</function> to exactly reconstruct the original
data. <function>decompress</function> is not necessarily equivalent
to <function>fetch</function>, either, since the other GiST methods might not
require full reconstruction of the data.)
</para>
<para>
The <acronym>SQL</acronym> declaration of the function must look like this:
<programlisting>
CREATE OR REPLACE FUNCTION my_decompress(internal)
RETURNS internal
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;
</programlisting>
And the matching code in the C module could then follow this skeleton:
<programlisting>
PG_FUNCTION_INFO_V1(my_decompress);
Datum
my_decompress(PG_FUNCTION_ARGS)
{
PG_RETURN_POINTER(PG_GETARG_POINTER(0));
}
</programlisting>
The above skeleton is suitable for the case where no decompression
is needed. (But, of course, omitting the method altogether is even
easier, and is recommended in such cases.)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>penalty</function></term>
<listitem>
<para>
Returns a value indicating the <quote>cost</quote> of inserting the new
entry into a particular branch of the tree. Items will be inserted
down the path of least <function>penalty</function> in the tree.
Values returned by <function>penalty</function> should be non-negative.
If a negative value is returned, it will be treated as zero.
</para>
<para>
The <acronym>SQL</acronym> declaration of the function must look like this:
<programlisting>
CREATE OR REPLACE FUNCTION my_penalty(internal, internal, internal)
RETURNS internal
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT; -- in some cases penalty functions need not be strict
</programlisting>
And the matching code in the C module could then follow this skeleton:
<programlisting>
PG_FUNCTION_INFO_V1(my_penalty);
Datum
my_penalty(PG_FUNCTION_ARGS)
{
GISTENTRY *origentry = (GISTENTRY *) PG_GETARG_POINTER(0);
GISTENTRY *newentry = (GISTENTRY *) PG_GETARG_POINTER(1);
float *penalty = (float *) PG_GETARG_POINTER(2);
data_type *orig = DatumGetDataType(origentry->key);
data_type *new = DatumGetDataType(newentry->key);
*penalty = my_penalty_implementation(orig, new);
PG_RETURN_POINTER(penalty);
}
</programlisting>
For historical reasons, the <function>penalty</function> function doesn't
just return a <type>float</type> result; instead it has to store the value
at the location indicated by the third argument. The return
value per se is ignored, though it's conventional to pass back the
address of that argument.
</para>
<para>
The <function>penalty</function> function is crucial to good performance of
the index. It'll get used at insertion time to determine which branch
to follow when choosing where to add the new entry in the tree. At
query time, the more balanced the index, the quicker the lookup.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>picksplit</function></term>
<listitem>
<para>
When an index 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.
</para>
<para>
The <acronym>SQL</acronym> declaration of the function must look like this:
<programlisting>
CREATE OR REPLACE FUNCTION my_picksplit(internal, internal)
RETURNS internal
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;
</programlisting>
And the matching code in the C module could then follow this skeleton:
<programlisting>
PG_FUNCTION_INFO_V1(my_picksplit);
Datum
my_picksplit(PG_FUNCTION_ARGS)
{
GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0);
GIST_SPLITVEC *v = (GIST_SPLITVEC *) PG_GETARG_POINTER(1);
OffsetNumber maxoff = entryvec->n - 1;
GISTENTRY *ent = entryvec->vector;
int i,
nbytes;
OffsetNumber *left,
*right;
data_type *tmp_union;
data_type *unionL;
data_type *unionR;
GISTENTRY **raw_entryvec;
maxoff = entryvec->n - 1;
nbytes = (maxoff + 1) * sizeof(OffsetNumber);
v->spl_left = (OffsetNumber *) palloc(nbytes);
left = v->spl_left;
v->spl_nleft = 0;
v->spl_right = (OffsetNumber *) palloc(nbytes);
right = v->spl_right;
v->spl_nright = 0;
unionL = NULL;
unionR = NULL;
/* Initialize the raw entry vector. */
raw_entryvec = (GISTENTRY **) malloc(entryvec->n * sizeof(void *));
for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i))
raw_entryvec[i] = &(entryvec->vector[i]);
for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i))
{
int real_index = raw_entryvec[i] - entryvec->vector;
tmp_union = DatumGetDataType(entryvec->vector[real_index].key);
Assert(tmp_union != NULL);
/*
* Choose where to put the index entries and update unionL and unionR
* accordingly. Append the entries to either v->spl_left or
* v->spl_right, and care about the counters.
*/
if (my_choice_is_left(unionL, curl, unionR, curr))
{
if (unionL == NULL)
unionL = tmp_union;
else
unionL = my_union_implementation(unionL, tmp_union);
*left = real_index;
++left;
++(v->spl_nleft);
}
else
{
/*
* Same on the right
*/
}
}
v->spl_ldatum = DataTypeGetDatum(unionL);
v->spl_rdatum = DataTypeGetDatum(unionR);
PG_RETURN_POINTER(v);
}
</programlisting>
Notice that the <function>picksplit</function> function's result is delivered
by modifying the passed-in <structname>v</structname> structure. The return
value per se is ignored, though it's conventional to pass back the
address of <structname>v</structname>.
</para>
<para>
Like <function>penalty</function>, the <function>picksplit</function> function
is crucial to good performance of the index. Designing suitable
<function>penalty</function> and <function>picksplit</function> implementations
is where the challenge of implementing well-performing
<acronym>GiST</acronym> indexes lies.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>same</function></term>
<listitem>
<para>
Returns true if two index entries are identical, false otherwise.
(An <quote>index entry</quote> is a value of the index's storage type,
not necessarily the original indexed column's type.)
</para>
<para>
The <acronym>SQL</acronym> declaration of the function must look like this:
<programlisting>
CREATE OR REPLACE FUNCTION my_same(storage_type, storage_type, internal)
RETURNS internal
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;
</programlisting>
And the matching code in the C module could then follow this skeleton:
<programlisting>
PG_FUNCTION_INFO_V1(my_same);
Datum
my_same(PG_FUNCTION_ARGS)
{
prefix_range *v1 = PG_GETARG_PREFIX_RANGE_P(0);
prefix_range *v2 = PG_GETARG_PREFIX_RANGE_P(1);
bool *result = (bool *) PG_GETARG_POINTER(2);
*result = my_eq(v1, v2);
PG_RETURN_POINTER(result);
}
</programlisting>
For historical reasons, the <function>same</function> function doesn't
just return a Boolean result; instead it has to store the flag
at the location indicated by the third argument. The return
value per se is ignored, though it's conventional to pass back the
address of that argument.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>distance</function></term>
<listitem>
<para>
Given an index entry <literal>p</literal> and a query value <literal>q</literal>,
this function determines the index entry's
<quote>distance</quote> from the query value. This function must be
supplied if the operator class contains any ordering operators.
A query using the ordering operator will be implemented by returning
index entries with the smallest <quote>distance</quote> values first,
so the results must be consistent with the operator's semantics.
For a leaf index entry the result just represents the distance to
the index entry; for an internal tree node, the result must be the
smallest distance that any child entry could have.
</para>
<para>
The <acronym>SQL</acronym> declaration of the function must look like this:
<programlisting>
CREATE OR REPLACE FUNCTION my_distance(internal, data_type, smallint, oid, internal)
RETURNS float8
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;
</programlisting>
And the matching code in the C module could then follow this skeleton:
<programlisting>
PG_FUNCTION_INFO_V1(my_distance);
Datum
my_distance(PG_FUNCTION_ARGS)
{
GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
data_type *query = PG_GETARG_DATA_TYPE_P(1);
StrategyNumber strategy = (StrategyNumber) PG_GETARG_UINT16(2);
/* Oid subtype = PG_GETARG_OID(3); */
/* bool *recheck = (bool *) PG_GETARG_POINTER(4); */
data_type *key = DatumGetDataType(entry->key);
double retval;
/*
* determine return value as a function of strategy, key and query.
*/
PG_RETURN_FLOAT8(retval);
}
</programlisting>
The arguments to the <function>distance</function> function are identical to
the arguments of the <function>consistent</function> function.
</para>
<para>
Some approximation is allowed when determining the distance, so long
as the result is never greater than the entry's actual distance. Thus,
for example, distance to a bounding box is usually sufficient in
geometric applications. For an internal tree node, the distance
returned must not be greater than the distance to any of the child
nodes. If the returned distance is not exact, the function must set
<literal>*recheck</literal> to true. (This is not necessary for internal tree
nodes; for them, the calculation is always assumed to be inexact.) In
this case the executor will calculate the accurate distance after
fetching the tuple from the heap, and reorder the tuples if necessary.
</para>
<para>
If the distance function returns <literal>*recheck = true</literal> for any
leaf node, the original ordering operator's return type must
be <type>float8</type> or <type>float4</type>, and the distance function's
result values must be comparable to those of the original ordering
operator, since the executor will sort using both distance function
results and recalculated ordering-operator results. Otherwise, the
distance function's result values can be any finite <type>float8</type>
values, so long as the relative order of the result values matches the
order returned by the ordering operator. (Infinity and minus infinity
are used internally to handle cases such as nulls, so it is not
recommended that <function>distance</function> functions return these values.)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>fetch</function></term>
<listitem>
<para>
Converts the compressed index representation of a data item into the
original data type, for index-only scans. The returned data must be an
exact, non-lossy copy of the originally indexed value.
</para>
<para>
The <acronym>SQL</acronym> declaration of the function must look like this:
<programlisting>
CREATE OR REPLACE FUNCTION my_fetch(internal)
RETURNS internal
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;
</programlisting>
The argument is a pointer to a <structname>GISTENTRY</structname> struct. On
entry, its <structfield>key</structfield> field contains a non-NULL leaf datum in
compressed form. The return value is another <structname>GISTENTRY</structname>
struct, whose <structfield>key</structfield> field contains the same datum in its
original, uncompressed form. If the opclass's compress function does
nothing for leaf entries, the <function>fetch</function> method can return the
argument as-is. Or, if the opclass does not have a compress function,
the <function>fetch</function> method can be omitted as well, since it would
necessarily be a no-op.
</para>
<para>
The matching code in the C module could then follow this skeleton:
<programlisting>
PG_FUNCTION_INFO_V1(my_fetch);
Datum
my_fetch(PG_FUNCTION_ARGS)
{
GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
input_data_type *in = DatumGetPointer(entry->key);
fetched_data_type *fetched_data;
GISTENTRY *retval;
retval = palloc(sizeof(GISTENTRY));
fetched_data = palloc(sizeof(fetched_data_type));
/*
* Convert 'fetched_data' into the a Datum of the original datatype.
*/
/* fill *retval from fetched_data. */
gistentryinit(*retval, PointerGetDatum(converted_datum),
entry->rel, entry->page, entry->offset, FALSE);
PG_RETURN_POINTER(retval);
}
</programlisting>
</para>
<para>
If the compress method is lossy for leaf entries, the operator class
cannot support index-only scans, and must not define
a <function>fetch</function> function.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>options</function></term>
<listitem>
<para>
Allows definition of user-visible parameters that control operator
class behavior.
</para>
<para>
The <acronym>SQL</acronym> declaration of the function must look like this:
<programlisting>
CREATE OR REPLACE FUNCTION my_options(internal)
RETURNS void
AS 'MODULE_PATHNAME'
LANGUAGE C STRICT;
</programlisting>
</para>
<para>
The function is passed a pointer to a <structname>local_relopts</structname>
struct, which needs to be filled with a set of operator class
specific options. The options can be accessed from other support
functions using the <literal>PG_HAS_OPCLASS_OPTIONS()</literal> and
<literal>PG_GET_OPCLASS_OPTIONS()</literal> macros.
</para>
<para>
An example implementation of my_options() and parameters use
from other support functions are given below:
<programlisting>
typedef enum MyEnumType
{
MY_ENUM_ON,
MY_ENUM_OFF,
MY_ENUM_AUTO
} MyEnumType;
typedef struct
{
int32 vl_len_; /* varlena header (do not touch directly!) */
int int_param; /* integer parameter */
double real_param; /* real parameter */
MyEnumType enum_param; /* enum parameter */
int str_param; /* string parameter */