491 lines
14 KiB
Plaintext
491 lines
14 KiB
Plaintext
<!-- doc/src/sgml/brin.sgml -->
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<chapter id="BRIN">
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<title>BRIN Indexes</title>
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<indexterm>
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<primary>index</primary>
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<secondary>BRIN</secondary>
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</indexterm>
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<sect1 id="brin-intro">
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<title>Introduction</title>
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<para>
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<acronym>BRIN</acronym> stands for Block Range Index.
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<acronym>BRIN</acronym> is designed for handling very large tables
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in which certain columns have some natural correlation with their
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physical location within the table.
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A <firstterm>block range</> is a group of pages that are physically
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adjacent in the table; for each block range, some summary info is stored
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by the index.
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For example, a table storing a store's sale orders might have
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a date column on which each order was placed, and most of the time
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the entries for earlier orders will appear earlier in the table as well;
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a table storing a ZIP code column might have all codes for a city
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grouped together naturally.
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</para>
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<para>
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<acronym>BRIN</acronym> indexes can satisfy queries via regular bitmap
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index scans, and will return all tuples in all pages within each range if
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the summary info stored by the index is <firstterm>consistent</> with the
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query conditions.
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The query executor is in charge of rechecking these tuples and discarding
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those that do not match the query conditions — in other words, these
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indexes are lossy.
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Because a <acronym>BRIN</acronym> index is very small, scanning the index
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adds little overhead compared to a sequential scan, but may avoid scanning
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large parts of the table that are known not to contain matching tuples.
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</para>
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<para>
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The specific data that a <acronym>BRIN</acronym> index will store,
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as well as the specific queries that the index will be able to satisfy,
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depend on the operator class selected for each column of the index.
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Data types having a linear sort order can have operator classes that
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store the minimum and maximum value within each block range, for instance;
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geometrical types might store the bounding box for all the objects
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in the block range.
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</para>
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<para>
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The size of the block range is determined at index creation time by
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the <literal>pages_per_range</> storage parameter. The number of index
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entries will be equal to the size of the relation in pages divided by
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the selected value for <literal>pages_per_range</>. Therefore, the smaller
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the number, the larger the index becomes (because of the need to
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store more index entries), but at the same time the summary data stored can
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be more precise and more data blocks can be skipped during an index scan.
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</para>
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</sect1>
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<sect1 id="brin-builtin-opclasses">
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<title>Built-in Operator Classes</title>
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<para>
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The core <productname>PostgreSQL</productname> distribution
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includes the <acronym>BRIN</acronym> operator classes shown in
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<xref linkend="brin-builtin-opclasses-table">.
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</para>
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<para>
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The <firstterm>minmax</>
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operator classes store the minimum and the maximum values appearing
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in the indexed column within the range.
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</para>
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<table id="brin-builtin-opclasses-table">
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<title>Built-in <acronym>BRIN</acronym> Operator Classes</title>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Name</entry>
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<entry>Indexed Data Type</entry>
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<entry>Indexable Operators</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><literal>bytea_minmax_ops</literal></entry>
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<entry><type>bytea</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>char_minmax_ops</literal></entry>
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<entry><type>"char"</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>name_minmax_ops</literal></entry>
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<entry><type>name</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>int8_minmax_ops</literal></entry>
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<entry><type>bigint</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>int2_minmax_ops</literal></entry>
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<entry><type>smallint</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>int4_minmax_ops</literal></entry>
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<entry><type>integer</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>text_minmax_ops</literal></entry>
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<entry><type>text</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>oid_minmax_ops</literal></entry>
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<entry><type>oid</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>tid_minmax_ops</literal></entry>
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<entry><type>tid</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>float4_minmax_ops</literal></entry>
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<entry><type>real</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>float8_minmax_ops</literal></entry>
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<entry><type>double precision</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>abstime_minmax_ops</literal></entry>
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<entry><type>abstime</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>reltime_minmax_ops</literal></entry>
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<entry><type>reltime</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>macaddr_minmax_ops</literal></entry>
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<entry><type>macaddr</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>inet_minmax_ops</literal></entry>
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<entry><type>inet</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>bpchar_minmax_ops</literal></entry>
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<entry><type>character</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>date_minmax_ops</literal></entry>
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<entry><type>date</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>time_minmax_ops</literal></entry>
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<entry><type>time without time zone</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>timestamp_minmax_ops</literal></entry>
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<entry><type>timestamp without time zone</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>timestamptz_minmax_ops</literal></entry>
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<entry><type>timestamp with time zone</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>interval_minmax_ops</literal></entry>
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<entry><type>interval</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>timetz_minmax_ops</literal></entry>
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<entry><type>time with time zone</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>bit_minmax_ops</literal></entry>
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<entry><type>bit</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>varbit_minmax_ops</literal></entry>
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<entry><type>bit varying</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>numeric_minmax_ops</literal></entry>
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<entry><type>numeric</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>uuid_minmax_ops</literal></entry>
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<entry><type>uuid</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</entry>
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</row>
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<row>
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<entry><literal>pg_lsn_minmax_ops</literal></entry>
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<entry><type>pg_lsn</type></entry>
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<entry>
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<literal><</literal>
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<literal><=</literal>
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<literal>=</literal>
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<literal>>=</literal>
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<literal>></literal>
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</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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</sect1>
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<sect1 id="brin-extensibility">
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<title>Extensibility</title>
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<para>
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The <acronym>BRIN</acronym> interface has a high level of abstraction,
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requiring the access method implementer only to implement the semantics
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of the data type being accessed. The <acronym>BRIN</acronym> layer
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itself takes care of concurrency, logging and searching the index structure.
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</para>
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<para>
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All it takes to get a <acronym>BRIN</acronym> access method working is to
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implement a few user-defined methods, which define the behavior of
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summary values stored in the index and the way they interact with
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scan keys.
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In short, <acronym>BRIN</acronym> combines
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extensibility with generality, code reuse, and a clean interface.
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</para>
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<para>
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There are four methods that an operator class for <acronym>BRIN</acronym>
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must provide:
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<variablelist>
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<varlistentry>
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<term><function>BrinOpcInfo *opcInfo(Oid type_oid)</></term>
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<listitem>
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<para>
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Returns internal information about the indexed columns' summary data.
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The return value must point to a palloc'd <structname>BrinOpcInfo</>,
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which has this definition:
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<programlisting>
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typedef struct BrinOpcInfo
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{
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/* Number of columns stored in an index column of this opclass */
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uint16 oi_nstored;
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/* Opaque pointer for the opclass' private use */
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void *oi_opaque;
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/* Type cache entries of the stored columns */
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TypeCacheEntry *oi_typcache[FLEXIBLE_ARRAY_MEMBER];
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} BrinOpcInfo;
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</programlisting>
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<structname>BrinOpcInfo</>.<structfield>oi_opaque</> can be used by the
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operator class routines to pass information between support procedures
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during an index scan.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><function>bool consistent(BrinDesc *bdesc, BrinValues *column,
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ScanKey key)</function></term>
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<listitem>
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<para>
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Returns whether the ScanKey is consistent with the given indexed
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values for a range.
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The attribute number to use is passed as part of the scan key.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><function>bool addValue(BrinDesc *bdesc, BrinValues *column,
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Datum newval, bool isnull)</function></term>
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<listitem>
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<para>
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Given an index tuple and an indexed value, modifies the indicated
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attribute of the tuple so that it additionally represents the new value.
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If any modification was done to the tuple, <literal>true</literal> is
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returned.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><function>bool unionTuples(BrinDesc *bdesc, BrinValues *a,
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BrinValues *b)</function></term>
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<listitem>
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<para>
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Consolidates two index tuples. Given two index tuples, modifies the
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indicated attribute of the first of them so that it represents both tuples.
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The second tuple is not modified.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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To implement these methods in a generic way, the operator class
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defines its own internal support functions.
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(For instance, <quote>min/max</> operator classes implements
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support functions for the four inequality operators for the data type.)
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Additionally, the operator class must supply appropriate
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operator entries,
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to enable the optimizer to use the index when those operators are
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used in queries.
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</para>
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</sect1>
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</chapter>
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