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Commitb576757

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Add external documentation for KNNGIST.
1 parent04910a3 commitb576757

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5 files changed

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‎doc/src/sgml/gist.sgml‎

Lines changed: 74 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -78,7 +78,7 @@
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<para>
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All it takes to get a <acronym>GiST</acronym> access method up and running
81-
is to implementseven user-defined methods, which define the behavior of
81+
is to implementseveral user-defined methods, which define the behavior of
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keys in the tree. Of course these methods have to be pretty fancy to
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support fancy queries, but for all the standard queries (B-trees,
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R-trees, etc.) they're relatively straightforward. In short,
@@ -93,19 +93,23 @@
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<para>
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There are seven methods that an index operator class for
96-
<acronym>GiST</acronym> must provide. Correctness of the index is ensured
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<acronym>GiST</acronym> must provide, and an eighth that is optional.
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Correctness of the index is ensured
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by proper implementation of the <function>same</>, <function>consistent</>
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and <function>union</> methods, while efficiency (size and speed) of the
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index will depend on the <function>penalty</> and <function>picksplit</>
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methods.
101-
The remaining two methods are <function>compress</> and
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The remaining twobasicmethods are <function>compress</> and
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<function>decompress</>, which allow an index to have internal tree data of
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a different type than the data it indexes. The leaves are to be of the
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indexed data type, while the other tree nodes can be of any C struct (but
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you still have to follow <productname>PostgreSQL</> data type rules here,
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see about <literal>varlena</> for variable sized data). If the tree's
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internal data type exists at the SQL level, the <literal>STORAGE</> option
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of the <command>CREATE OPERATOR CLASS</> command can be used.
110+
The optional eighth method is <function>distance</>, which is needed
111+
if the operator class wishes to support ordered scans (nearest-neighbor
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searches).
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</para>
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111115
<variablelist>
@@ -567,6 +571,73 @@ my_same(PG_FUNCTION_ARGS)
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><function>distance</></term>
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<listitem>
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<para>
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Given an index entry <literal>p</> and a query value <literal>q</>,
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this function determines the index entry's
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<quote>distance</> from the query value. This function must be
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supplied if the operator class contains any ordering operators.
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A query using the ordering operator will be implemented by returning
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index entries with the smallest <quote>distance</> values first,
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so the results must be consistent with the operator's semantics.
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For a leaf index entry the result just represents the distance to
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the index entry; for an internal tree node, the result must be the
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smallest distance that any child entry could have.
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</para>
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<para>
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The <acronym>SQL</> declaration of the function must look like this:
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<programlisting>
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CREATE OR REPLACE FUNCTION my_distance(internal, data_type, smallint, oid)
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RETURNS float8
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AS 'MODULE_PATHNAME'
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LANGUAGE C STRICT;
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</programlisting>
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600+
And the matching code in the C module could then follow this skeleton:
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602+
<programlisting>
603+
Datum my_distance(PG_FUNCTION_ARGS);
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PG_FUNCTION_INFO_V1(my_distance);
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Datum
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my_distance(PG_FUNCTION_ARGS)
608+
{
609+
GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
610+
data_type *query = PG_GETARG_DATA_TYPE_P(1);
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StrategyNumber strategy = (StrategyNumber) PG_GETARG_UINT16(2);
612+
/* Oid subtype = PG_GETARG_OID(3); */
613+
data_type *key = DatumGetDataType(entry-&gt;key);
614+
double retval;
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616+
/*
617+
* determine return value as a function of strategy, key and query.
618+
*/
619+
620+
PG_RETURN_FLOAT8(retval);
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}
622+
</programlisting>
623+
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The arguments to the <function>distance</> function are identical to
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the arguments of the <function>consistent</> function, except that no
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recheck flag is used. The distance to a leaf index entry must always
627+
be determined exactly, since there is no way to re-order the tuples
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once they are returned. Some approximation is allowed when determining
629+
the distance to an internal tree node, so long as the result is never
630+
greater than any child's actual distance. Thus, for example, distance
631+
to a bounding box is usually sufficient in geometric applications. The
632+
result value can be any finite <type>float8</> value. (Infinity and
633+
minus infinity are used internally to handle cases such as nulls, so it
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is not recommended that <function>distance</> functions return these
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values.)
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect1>

‎doc/src/sgml/indexam.sgml‎

Lines changed: 28 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -505,11 +505,31 @@ amrestrpos (IndexScanDesc scan);
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<para>
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Some access methods return index entries in a well-defined order, others
508-
do not. If entries are returned in sorted order, the access method should
509-
set <structname>pg_am</>.<structfield>amcanorder</> true to indicate that
510-
it supports ordered scans.
511-
All such access methods must use btree-compatible strategy numbers for
512-
their equality and ordering operators.
508+
do not. There are actually two different ways that an access method can
509+
support sorted output:
510+
511+
<itemizedlist>
512+
<listitem>
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<para>
514+
Access methods that always return entries in the natural ordering
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of their data (such as btree) should set
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<structname>pg_am</>.<structfield>amcanorder</> to true.
517+
Currently, such access methods must use btree-compatible strategy
518+
numbers for their equality and ordering operators.
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</para>
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</listitem>
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<listitem>
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<para>
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Access methods that support ordering operators should set
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<structname>pg_am</>.<structfield>amcanorderbyop</> to true.
525+
This indicates that the index is capable of returning entries in
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an order satisfying <literal>ORDER BY</> <replaceable>index_key</>
527+
<replaceable>operator</> <replaceable>constant</>. Scan modifiers
528+
of that form can be passed to <function>amrescan</> as described
529+
previously.
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</para>
531+
</listitem>
532+
</itemizedlist>
513533
</para>
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515535
<para>
@@ -521,7 +541,7 @@ amrestrpos (IndexScanDesc scan);
521541
the normal front-to-back direction, so <function>amgettuple</> must return
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the last matching tuple in the index, rather than the first one as it
523543
normally would. (This will only occur for access
524-
methods thatadvertise they support ordered scans.) After the
544+
methods thatset <structfield>amcanorder</> to true.) After the
525545
first call, <function>amgettuple</> must be prepared to advance the scan in
526546
either direction from the most recently returned entry. (But if
527547
<structname>pg_am</>.<structfield>amcanbackward</> is false, all subsequent
@@ -563,7 +583,8 @@ amrestrpos (IndexScanDesc scan);
563583
tuples at once and marking or restoring scan positions isn't
564584
supported. Secondly, the tuples are returned in a bitmap which doesn't
565585
have any specific ordering, which is why <function>amgetbitmap</> doesn't
566-
take a <literal>direction</> argument. Finally, <function>amgetbitmap</>
586+
take a <literal>direction</> argument. (Ordering operators will never be
587+
supplied for such a scan, either.) Finally, <function>amgetbitmap</>
567588
does not guarantee any locking of the returned tuples, with implications
568589
spelled out in <xref linkend="index-locking">.
569590
</para>

‎doc/src/sgml/indices.sgml‎

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Original file line numberDiff line numberDiff line change
@@ -167,6 +167,11 @@ CREATE INDEX test1_id_index ON test1 (id);
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upper/lower case conversion.
168168
</para>
169169

170+
<para>
171+
B-tree indexes can also be used to retrieve data in sorted order.
172+
This is not always faster than a simple scan and sort, but it is
173+
often helpful.
174+
</para>
170175

171176
<para>
172177
<indexterm>
@@ -236,6 +241,18 @@ CREATE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable>
236241
classes are available in the <literal>contrib</> collection or as separate
237242
projects. For more information see <xref linkend="GiST">.
238243
</para>
244+
245+
<para>
246+
GiST indexes are also capable of optimizing <quote>nearest-neighbor</>
247+
searches, such as
248+
<programlisting><![CDATA[
249+
SELECT * FROM places ORDER BY location <-> point '(101,456)' LIMIT 10;
250+
]]>
251+
</programlisting>
252+
which finds the ten places closest to a given target point. The ability
253+
to do this is again dependent on the particular operator class being used.
254+
</para>
255+
239256
<para>
240257
<indexterm>
241258
<primary>index</primary>

‎doc/src/sgml/xindex.sgml‎

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@@ -361,59 +361,74 @@
361361
</table>
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<para>
364-
GiST indexes require seven support functions,
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GiST indexes require seven support functions, with an optional eighth, as
365365
shown in <xref linkend="xindex-gist-support-table">.
366366
</para>
367367

368368
<table tocentry="1" id="xindex-gist-support-table">
369369
<title>GiST Support Functions</title>
370-
<tgroup cols="2">
370+
<tgroup cols="3">
371371
<thead>
372372
<row>
373373
<entry>Function</entry>
374+
<entry>Description</entry>
374375
<entry>Support Number</entry>
375376
</row>
376377
</thead>
377378
<tbody>
378379
<row>
379-
<entry>consistent - determine whether key satisfies the
380+
<entry><function>consistent</></entry>
381+
<entry>determine whether key satisfies the
380382
query qualifier</entry>
381383
<entry>1</entry>
382384
</row>
383385
<row>
384-
<entry>union - compute union of a set of keys</entry>
386+
<entry><function>union</></entry>
387+
<entry>compute union of a set of keys</entry>
385388
<entry>2</entry>
386389
</row>
387390
<row>
388-
<entry>compress - compute a compressed representation of a key or value
391+
<entry><function>compress</></entry>
392+
<entry>compute a compressed representation of a key or value
389393
to be indexed</entry>
390394
<entry>3</entry>
391395
</row>
392396
<row>
393-
<entry>decompress - compute a decompressed representation of a
397+
<entry><function>decompress</></entry>
398+
<entry>compute a decompressed representation of a
394399
compressed key</entry>
395400
<entry>4</entry>
396401
</row>
397402
<row>
398-
<entry>penalty - compute penalty for inserting new key into subtree
403+
<entry><function>penalty</></entry>
404+
<entry>compute penalty for inserting new key into subtree
399405
with given subtree's key</entry>
400406
<entry>5</entry>
401407
</row>
402408
<row>
403-
<entry>picksplit - determine which entries of a page are to be moved
409+
<entry><function>picksplit</></entry>
410+
<entry>determine which entries of a page are to be moved
404411
to the new page and compute the union keys for resulting pages</entry>
405412
<entry>6</entry>
406413
</row>
407414
<row>
408-
<entry>equal - compare two keys and return true if they are equal</entry>
415+
<entry><function>equal</></entry>
416+
<entry>compare two keys and return true if they are equal</entry>
409417
<entry>7</entry>
410418
</row>
419+
<row>
420+
<entry><function>distance</></entry>
421+
<entry>
422+
(optional method) determine distance from key to query value
423+
</entry>
424+
<entry>8</entry>
425+
</row>
411426
</tbody>
412427
</tgroup>
413428
</table>
414429

415430
<para>
416-
GIN indexes require four support functions,
431+
GIN indexes require four support functions, with an optional fifth, as
417432
shown in <xref linkend="xindex-gin-support-table">.
418433
</para>
419434

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