785 lines
26 KiB
Plaintext
785 lines
26 KiB
Plaintext
<!--
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$PostgreSQL: pgsql/doc/src/sgml/xindex.sgml,v 1.36 2003/11/29 19:51:38 pgsql Exp $
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-->
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<sect1 id="xindex">
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<title>Interfacing Extensions To Indexes</title>
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<indexterm zone="xindex">
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<primary>index</primary>
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<secondary>for user-defined data type</secondary>
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</indexterm>
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<para>
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The procedures described thus far let you define new types, new
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functions, and new operators. However, we cannot yet define an
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index on a column of a new data type. To do this, we must define an
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<firstterm>operator class</> for the new data type. Later in this
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section, we will illustrate this concept in an example: a new
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operator class for the B-tree index method that stores and sorts
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complex numbers in ascending absolute value order.
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</para>
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<note>
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<para>
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Prior to <productname>PostgreSQL</productname> release 7.3, it was
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necessary to make manual additions to the system catalogs
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<classname>pg_amop</>, <classname>pg_amproc</>, and
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<classname>pg_opclass</> in order to create a user-defined
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operator class. That approach is now deprecated in favor of
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using <command>CREATE OPERATOR CLASS</>, which is a much simpler
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and less error-prone way of creating the necessary catalog entries.
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</para>
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</note>
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<sect2 id="xindex-im">
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<title>Index Methods and Operator Classes</title>
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<para>
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The <classname>pg_am</classname> table contains one row for every
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index method (internally known as access method). Support for
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regular access to tables is built into
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<productname>PostgreSQL</productname>, but all index methods are
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described in <classname>pg_am</classname>. It is possible to add a
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new index method by defining the required interface routines and
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then creating a row in <classname>pg_am</classname> --- but that is
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far beyond the scope of this chapter.
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</para>
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<para>
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The routines for an index method do not directly know anything
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about the data types that the index method will operate on.
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Instead, an <firstterm>operator
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class</><indexterm><primary>operator class</></indexterm>
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identifies the set of operations that the index method needs to use
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to work with a particular data type. Operator classes are so
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called because one thing they specify is the set of
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<literal>WHERE</>-clause operators that can be used with an index
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(i.e., can be converted into an index-scan qualification). An
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operator class may also specify some <firstterm>support
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procedures</> that are needed by the internal operations of the
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index method, but do not directly correspond to any
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<literal>WHERE</>-clause operator that can be used with the index.
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</para>
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<para>
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It is possible to define multiple operator classes for the same
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data type and index method. By doing this, multiple
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sets of indexing semantics can be defined for a single data type.
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For example, a B-tree index requires a sort ordering to be defined
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for each data type it works on.
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It might be useful for a complex-number data type
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to have one B-tree operator class that sorts the data by complex
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absolute value, another that sorts by real part, and so on.
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Typically, one of the operator classes will be deemed most commonly
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useful and will be marked as the default operator class for that
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data type and index method.
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</para>
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<para>
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The same operator class name
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can be used for several different index methods (for example, both B-tree
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and hash index methods have operator classes named
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<literal>int4_ops</literal>), but each such class is an independent
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entity and must be defined separately.
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</para>
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</sect2>
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<sect2 id="xindex-strategies">
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<title>Index Method Strategies</title>
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<para>
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The operators associated with an operator class are identified by
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<quote>strategy numbers</>, which serve to identify the semantics of
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each operator within the context of its operator class.
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For example, B-trees impose a strict ordering on keys, lesser to greater,
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and so operators like <quote>less than</> and <quote>greater than or equal
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to</> are interesting with respect to a B-tree.
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Because
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<productname>PostgreSQL</productname> allows the user to define operators,
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<productname>PostgreSQL</productname> cannot look at the name of an operator
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(e.g., <literal><</> or <literal>>=</>) and tell what kind of
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comparison it is. Instead, the index method defines a set of
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<quote>strategies</>, which can be thought of as generalized operators.
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Each operator class specifies which actual operator corresponds to each
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strategy for a particular data type and interpretation of the index
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semantics.
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</para>
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<para>
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The B-tree index method defines five strategies, shown in <xref
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linkend="xindex-btree-strat-table">.
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</para>
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<table tocentry="1" id="xindex-btree-strat-table">
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<title>B-tree Strategies</title>
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<tgroup cols="2">
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<thead>
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<row>
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<entry>Operation</entry>
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<entry>Strategy Number</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>less than</entry>
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<entry>1</entry>
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</row>
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<row>
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<entry>less than or equal</entry>
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<entry>2</entry>
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</row>
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<row>
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<entry>equal</entry>
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<entry>3</entry>
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</row>
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<row>
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<entry>greater than or equal</entry>
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<entry>4</entry>
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</row>
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<row>
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<entry>greater than</entry>
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<entry>5</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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Hash indexes express only bitwise equality, and so they use only one
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strategy, shown in <xref linkend="xindex-hash-strat-table">.
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</para>
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<table tocentry="1" id="xindex-hash-strat-table">
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<title>Hash Strategies</title>
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<tgroup cols="2">
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<thead>
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<row>
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<entry>Operation</entry>
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<entry>Strategy Number</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>equal</entry>
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<entry>1</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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R-tree indexes express rectangle-containment relationships.
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They use eight strategies, shown in <xref linkend="xindex-rtree-strat-table">.
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</para>
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<table tocentry="1" id="xindex-rtree-strat-table">
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<title>R-tree Strategies</title>
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<tgroup cols="2">
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<thead>
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<row>
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<entry>Operation</entry>
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<entry>Strategy Number</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>left of</entry>
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<entry>1</entry>
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</row>
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<row>
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<entry>left of or overlapping</entry>
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<entry>2</entry>
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</row>
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<row>
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<entry>overlapping</entry>
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<entry>3</entry>
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</row>
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<row>
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<entry>right of or overlapping</entry>
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<entry>4</entry>
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</row>
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<row>
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<entry>right of</entry>
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<entry>5</entry>
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</row>
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<row>
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<entry>same</entry>
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<entry>6</entry>
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</row>
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<row>
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<entry>contains</entry>
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<entry>7</entry>
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</row>
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<row>
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<entry>contained by</entry>
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<entry>8</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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GiST indexes are even more flexible: they do not have a fixed set of
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strategies at all. Instead, the <quote>consistency</> support routine
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of each particular GiST operator class interprets the strategy numbers
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however it likes.
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</para>
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<para>
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Note that all strategy operators return Boolean values. In
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practice, all operators defined as index method strategies must
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return type <type>boolean</type>, since they must appear at the top
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level of a <literal>WHERE</> clause to be used with an index.
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</para>
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<para>
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By the way, the <structfield>amorderstrategy</structfield> column
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in <classname>pg_am</> tells whether
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the index method supports ordered scans. Zero means it doesn't; if it
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does, <structfield>amorderstrategy</structfield> is the strategy
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number that corresponds to the ordering operator. For example, B-tree
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has <structfield>amorderstrategy</structfield> = 1, which is its
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<quote>less than</quote> strategy number.
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</para>
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</sect2>
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<sect2 id="xindex-support">
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<title>Index Method Support Routines</title>
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<para>
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Strategies aren't usually enough information for the system to figure
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out how to use an index. In practice, the index methods require
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additional support routines in order to work. For example, the B-tree
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index method must be able to compare two keys and determine whether one
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is greater than, equal to, or less than the other. Similarly, the
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R-tree index method must be able to compute
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intersections, unions, and sizes of rectangles. These
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operations do not correspond to operators used in qualifications in
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SQL commands; they are administrative routines used by
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the index methods, internally.
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</para>
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<para>
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Just as with strategies, the operator class identifies which specific
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functions should play each of these roles for a given data type and
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semantic interpretation. The index method defines the set
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of functions it needs, and the operator class identifies the correct
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functions to use by assigning them to the <quote>support function numbers</>.
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</para>
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<para>
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B-trees require a single support function, shown in <xref
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linkend="xindex-btree-support-table">.
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</para>
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<table tocentry="1" id="xindex-btree-support-table">
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<title>B-tree Support Functions</title>
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<tgroup cols="2">
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<thead>
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<row>
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<entry>Function</entry>
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<entry>Support Number</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>
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Compare two keys and return an integer less than zero, zero, or
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greater than zero, indicating whether the first key is less than, equal to,
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or greater than the second.
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</entry>
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<entry>1</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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Hash indexes likewise require one support function, shown in <xref
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linkend="xindex-hash-support-table">.
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</para>
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<table tocentry="1" id="xindex-hash-support-table">
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<title>Hash Support Functions</title>
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<tgroup cols="2">
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<thead>
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<row>
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<entry>Function</entry>
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<entry>Support Number</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>Compute the hash value for a key</entry>
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<entry>1</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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R-tree indexes require three support functions,
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shown in <xref linkend="xindex-rtree-support-table">.
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</para>
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<table tocentry="1" id="xindex-rtree-support-table">
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<title>R-tree Support Functions</title>
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<tgroup cols="2">
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<thead>
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<row>
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<entry>Function</entry>
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<entry>Support Number</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>union</entry>
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<entry>1</entry>
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</row>
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<row>
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<entry>intersection</entry>
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<entry>2</entry>
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</row>
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<row>
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<entry>size</entry>
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<entry>3</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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GiST indexes require seven support functions,
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shown in <xref linkend="xindex-gist-support-table">.
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</para>
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<table tocentry="1" id="xindex-gist-support-table">
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<title>GiST Support Functions</title>
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<tgroup cols="2">
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<thead>
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<row>
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<entry>Function</entry>
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<entry>Support Number</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>consistent</entry>
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<entry>1</entry>
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</row>
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<row>
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<entry>union</entry>
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<entry>2</entry>
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</row>
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<row>
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<entry>compress</entry>
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<entry>3</entry>
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</row>
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<row>
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<entry>decompress</entry>
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<entry>4</entry>
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</row>
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<row>
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<entry>penalty</entry>
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<entry>5</entry>
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</row>
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<row>
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<entry>picksplit</entry>
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<entry>6</entry>
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</row>
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<row>
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<entry>equal</entry>
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<entry>7</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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Unlike strategy operators, support functions return whichever data
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type the particular index method expects, for example in the case
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of the comparison function for B-trees, a signed integer.
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</para>
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</sect2>
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<sect2 id="xindex-example">
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<title>An Example</title>
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<para>
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Now that we have seen the ideas, here is the promised example of
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creating a new operator class.
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(You can find a working copy of this example in
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<filename>src/tutorial/complex.c</filename> and
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<filename>src/tutorial/complex.sql</filename> in the source
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distribution.)
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The operator class encapsulates
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operators that sort complex numbers in absolute value order, so we
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choose the name <literal>complex_abs_ops</literal>. First, we need
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a set of operators. The procedure for defining operators was
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discussed in <xref linkend="xoper">. For an operator class on
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B-trees, the operators we require are:
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<itemizedlist spacing="compact">
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<listitem><simpara>absolute-value less-than (strategy 1)</></>
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<listitem><simpara>absolute-value less-than-or-equal (strategy 2)</></>
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<listitem><simpara>absolute-value equal (strategy 3)</></>
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<listitem><simpara>absolute-value greater-than-or-equal (strategy 4)</></>
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<listitem><simpara>absolute-value greater-than (strategy 5)</></>
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</itemizedlist>
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</para>
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<para>
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The least error-prone way to define a related set of comparison operators
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is to write the B-tree comparison support function first, and then write the
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other functions as one-line wrappers around the support function. This
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reduces the odds of getting inconsistent results for corner cases.
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Following this approach, we first write
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<programlisting>
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#define Mag(c) ((c)->x*(c)->x + (c)->y*(c)->y)
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static int
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complex_abs_cmp_internal(Complex *a, Complex *b)
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{
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double amag = Mag(a),
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bmag = Mag(b);
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if (amag < bmag)
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return -1;
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if (amag > bmag)
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return 1;
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return 0;
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}
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</programlisting>
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Now the less-than function looks like
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<programlisting>
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PG_FUNCTION_INFO_V1(complex_abs_lt);
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Datum
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complex_abs_lt(PG_FUNCTION_ARGS)
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{
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Complex *a = (Complex *) PG_GETARG_POINTER(0);
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Complex *b = (Complex *) PG_GETARG_POINTER(1);
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PG_RETURN_BOOL(complex_abs_cmp_internal(a, b) < 0);
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}
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</programlisting>
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The other four functions differ only in how they compare the internal
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function's result to zero.
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</para>
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<para>
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Next we declare the functions and the operators based on the functions
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to SQL:
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<programlisting>
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CREATE FUNCTION complex_abs_lt(complex, complex) RETURNS bool
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AS '<replaceable>filename</replaceable>', 'complex_abs_lt'
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LANGUAGE C IMMUTABLE STRICT;
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CREATE OPERATOR < (
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leftarg = complex, rightarg = complex, procedure = complex_abs_lt,
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commutator = > , negator = >= ,
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restrict = scalarltsel, join = scalarltjoinsel
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);
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</programlisting>
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It is important to specify the correct commutator and negator operators,
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as well as suitable restriction and join selectivity
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functions, otherwise the optimizer will be unable to make effective
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use of the index. Note that the less-than, equal, and
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greater-than cases should use different selectivity functions.
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</para>
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<para>
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Other things worth noting are happening here:
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<itemizedlist>
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<listitem>
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<para>
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There can only be one operator named, say, <literal>=</literal>
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and taking type <type>complex</type> for both operands. In this
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case we don't have any other operator <literal>=</literal> for
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<type>complex</type>, but if we were building a practical data
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type we'd probably want <literal>=</literal> to be the ordinary
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equality operation for complex numbers (and not the equality of
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the absolute values). In that case, we'd need to use some other
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operator name for <function>complex_abs_eq</>.
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</para>
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</listitem>
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<listitem>
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<para>
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Although <productname>PostgreSQL</productname> can cope with
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functions having the same name as long as they have different
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argument data types, C can only cope with one global function
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having a given name. So we shouldn't name the C function
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something simple like <filename>abs_eq</filename>. Usually it's
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a good practice to include the data type name in the C function
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name, so as not to conflict with functions for other data types.
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</para>
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</listitem>
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<listitem>
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<para>
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We could have made the <productname>PostgreSQL</productname> name
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of the function <filename>abs_eq</filename>, relying on
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<productname>PostgreSQL</productname> to distinguish it by
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argument data types from any other
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<productname>PostgreSQL</productname> function of the same name.
|
|
To keep the example simple, we make the function have the same
|
|
names at the C level and <productname>PostgreSQL</productname>
|
|
level.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</para>
|
|
|
|
<para>
|
|
The next step is the registration of the support routine required
|
|
by B-trees. The example C code that implements this is in the same
|
|
file that contains the operator functions. This is how we declare
|
|
the function:
|
|
|
|
<programlisting>
|
|
CREATE FUNCTION complex_abs_cmp(complex, complex)
|
|
RETURNS integer
|
|
AS '<replaceable>filename</replaceable>'
|
|
LANGUAGE C IMMUTABLE STRICT;
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
Now that we have the required operators and support routine,
|
|
we can finally create the operator class:
|
|
|
|
<programlisting>
|
|
CREATE OPERATOR CLASS complex_abs_ops
|
|
DEFAULT FOR TYPE complex USING btree AS
|
|
OPERATOR 1 < ,
|
|
OPERATOR 2 <= ,
|
|
OPERATOR 3 = ,
|
|
OPERATOR 4 >= ,
|
|
OPERATOR 5 > ,
|
|
FUNCTION 1 complex_abs_cmp(complex, complex);
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
And we're done! It should now be possible to create
|
|
and use B-tree indexes on <type>complex</type> columns.
|
|
</para>
|
|
|
|
<para>
|
|
We could have written the operator entries more verbosely, as in
|
|
<programlisting>
|
|
OPERATOR 1 < (complex, complex) ,
|
|
</programlisting>
|
|
but there is no need to do so when the operators take the same data type
|
|
we are defining the operator class for.
|
|
</para>
|
|
|
|
<para>
|
|
The above example assumes that you want to make this new operator class the
|
|
default B-tree operator class for the <type>complex</type> data type.
|
|
If you don't, just leave out the word <literal>DEFAULT</>.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="xindex-opclass-crosstype">
|
|
<title>Cross-Data-Type Operator Classes</title>
|
|
|
|
<para>
|
|
So far we have implicitly assumed that an operator class deals with
|
|
only one data type. While there certainly can be only one data type in
|
|
a particular index column, it is often useful to index operations that
|
|
compare an indexed column to a value of a different data type. This is
|
|
presently supported by the B-tree and GiST index methods.
|
|
</para>
|
|
|
|
<para>
|
|
B-trees require the left-hand operand of each operator to be the indexed
|
|
data type, but the right-hand operand can be of a different type. There
|
|
must be a support function having a matching signature. For example,
|
|
the built-in operator class for type <type>bigint</> (<type>int8</>)
|
|
allows cross-type comparisons to <type>int4</> and <type>int2</>. It
|
|
could be duplicated by this definition:
|
|
|
|
<programlisting>
|
|
CREATE OPERATOR CLASS int8_ops
|
|
DEFAULT FOR TYPE int8 USING btree AS
|
|
-- standard int8 comparisons
|
|
OPERATOR 1 < ,
|
|
OPERATOR 2 <= ,
|
|
OPERATOR 3 = ,
|
|
OPERATOR 4 >= ,
|
|
OPERATOR 5 > ,
|
|
FUNCTION 1 btint8cmp(int8, int8) ,
|
|
|
|
-- cross-type comparisons to int2 (smallint)
|
|
OPERATOR 1 < (int8, int2) ,
|
|
OPERATOR 2 <= (int8, int2) ,
|
|
OPERATOR 3 = (int8, int2) ,
|
|
OPERATOR 4 >= (int8, int2) ,
|
|
OPERATOR 5 > (int8, int2) ,
|
|
FUNCTION 1 btint82cmp(int8, int2) ,
|
|
|
|
-- cross-type comparisons to int4 (integer)
|
|
OPERATOR 1 < (int8, int4) ,
|
|
OPERATOR 2 <= (int8, int4) ,
|
|
OPERATOR 3 = (int8, int4) ,
|
|
OPERATOR 4 >= (int8, int4) ,
|
|
OPERATOR 5 > (int8, int4) ,
|
|
FUNCTION 1 btint84cmp(int8, int4) ;
|
|
</programlisting>
|
|
|
|
Notice that this definition <quote>overloads</> the operator strategy and
|
|
support function numbers. This is allowed (for B-tree operator classes
|
|
only) so long as each instance of a particular number has a different
|
|
right-hand data type. The instances that are not cross-type are the
|
|
default or primary operators of the operator class.
|
|
</para>
|
|
|
|
<para>
|
|
GiST indexes do not allow overloading of strategy or support function
|
|
numbers, but it is still possible to get the effect of supporting
|
|
multiple right-hand data types, by assigning a distinct strategy number
|
|
to each operator that needs to be supported. The <literal>consistent</>
|
|
support function must determine what it needs to do based on the strategy
|
|
number, and must be prepared to accept comparison values of the appropriate
|
|
data types.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2 id="xindex-opclass-dependencies">
|
|
<title>System Dependencies on Operator Classes</title>
|
|
|
|
<indexterm>
|
|
<primary>ordering operator</primary>
|
|
</indexterm>
|
|
|
|
<para>
|
|
<productname>PostgreSQL</productname> uses operator classes to infer the
|
|
properties of operators in more ways than just whether they can be used
|
|
with indexes. Therefore, you might want to create operator classes
|
|
even if you have no intention of indexing any columns of your data type.
|
|
</para>
|
|
|
|
<para>
|
|
In particular, there are SQL features such as <literal>ORDER BY</> and
|
|
<literal>DISTINCT</> that require comparison and sorting of values.
|
|
To implement these features on a user-defined data type,
|
|
<productname>PostgreSQL</productname> looks for the default B-tree operator
|
|
class for the data type. The <quote>equals</> member of this operator
|
|
class defines the system's notion of equality of values for
|
|
<literal>GROUP BY</> and <literal>DISTINCT</>, and the sort ordering
|
|
imposed by the operator class defines the default <literal>ORDER BY</>
|
|
ordering.
|
|
</para>
|
|
|
|
<para>
|
|
Comparison of arrays of user-defined types also relies on the semantics
|
|
defined by the default B-tree operator class.
|
|
</para>
|
|
|
|
<para>
|
|
If there is no default B-tree operator class for a data type, the system
|
|
will look for a default hash operator class. But since that kind of
|
|
operator class only provides equality, in practice it is only enough
|
|
to support array equality.
|
|
</para>
|
|
|
|
<para>
|
|
When there is no default operator class for a data type, you will get
|
|
errors like <quote>could not identify an ordering operator</> if you
|
|
try to use these SQL features with the data type.
|
|
</para>
|
|
|
|
<note>
|
|
<para>
|
|
In <ProductName>PostgreSQL</ProductName> versions before 7.4,
|
|
sorting and grouping operations would implicitly use operators named
|
|
<literal>=</>, <literal><</>, and <literal>></>. The new
|
|
behavior of relying on default operator classes avoids having to make
|
|
any assumption about the behavior of operators with particular names.
|
|
</para>
|
|
</note>
|
|
</sect2>
|
|
|
|
<sect2 id="xindex-opclass-features">
|
|
<title>Special Features of Operator Classes</title>
|
|
|
|
<para>
|
|
There are two special features of operator classes that we have
|
|
not discussed yet, mainly because they are not useful
|
|
with the most commonly used index methods.
|
|
</para>
|
|
|
|
<para>
|
|
Normally, declaring an operator as a member of an operator class means
|
|
that the index method can retrieve exactly the set of rows
|
|
that satisfy a <literal>WHERE</> condition using the operator. For example,
|
|
<programlisting>
|
|
SELECT * FROM table WHERE integer_column < 4;
|
|
</programlisting>
|
|
can be satisfied exactly by a B-tree index on the integer column.
|
|
But there are cases where an index is useful as an inexact guide to
|
|
the matching rows. For example, if an R-tree index stores only
|
|
bounding boxes for objects, then it cannot exactly satisfy a <literal>WHERE</>
|
|
condition that tests overlap between nonrectangular objects such as
|
|
polygons. Yet we could use the index to find objects whose bounding
|
|
box overlaps the bounding box of the target object, and then do the
|
|
exact overlap test only on the objects found by the index. If this
|
|
scenario applies, the index is said to be <quote>lossy</> for the
|
|
operator, and we add <literal>RECHECK</> to the <literal>OPERATOR</> clause
|
|
in the <command>CREATE OPERATOR CLASS</> command.
|
|
<literal>RECHECK</> is valid if the index is guaranteed to return
|
|
all the required rows, plus perhaps some additional rows, which
|
|
can be eliminated by performing the original operator invocation.
|
|
</para>
|
|
|
|
<para>
|
|
Consider again the situation where we are storing in the index only
|
|
the bounding box of a complex object such as a polygon. In this
|
|
case there's not much value in storing the whole polygon in the index
|
|
entry --- we may as well store just a simpler object of type
|
|
<type>box</>. This situation is expressed by the <literal>STORAGE</>
|
|
option in <command>CREATE OPERATOR CLASS</>: we'd write something like
|
|
|
|
<programlisting>
|
|
CREATE OPERATOR CLASS polygon_ops
|
|
DEFAULT FOR TYPE polygon USING gist AS
|
|
...
|
|
STORAGE box;
|
|
</programlisting>
|
|
|
|
At present, only the GiST index method supports a
|
|
<literal>STORAGE</> type that's different from the column data type.
|
|
The GiST <literal>compress</> and <literal>decompress</> support
|
|
routines must deal with data-type conversion when <literal>STORAGE</>
|
|
is used.
|
|
</para>
|
|
</sect2>
|
|
|
|
</sect1>
|
|
|
|
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|
|
Local variables:
|
|
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|
|
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|