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Green_GoodERP18_FSJ_Standard/PostgreSQL/doc/postgresql/html/spgist-implementation.html

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  1. <?xml version="1.0" encoding="UTF-8" standalone="no"?>

  2. <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>65.4. Implementation</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="spgist-extensibility.html" title="65.3. Extensibility" /><link rel="next" href="spgist-examples.html" title="65.5. Examples" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">65.4. Implementation</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="spgist-extensibility.html" title="65.3. Extensibility">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="spgist.html" title="Chapter 65. SP-GiST Indexes">Up</a></td><th width="60%" align="center">Chapter 65. SP-GiST Indexes</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="spgist-examples.html" title="65.5. Examples">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="SPGIST-IMPLEMENTATION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">65.4. Implementation</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="spgist-implementation.html#SPGIST-LIMITS">65.4.1. SP-GiST Limits</a></span></dt><dt><span class="sect2"><a href="spgist-implementation.html#SPGIST-NULL-LABELS">65.4.2. SP-GiST Without Node Labels</a></span></dt><dt><span class="sect2"><a href="spgist-implementation.html#SPGIST-ALL-THE-SAME">65.4.3. <span class="quote">“<span class="quote">All-the-Same</span>”</span> Inner Tuples</a></span></dt></dl></div><p>

  3.    This section covers implementation details and other tricks that are

  4.    useful for implementers of <acronym class="acronym">SP-GiST</acronym> operator classes to

  5.    know.

  6.   </p><div class="sect2" id="SPGIST-LIMITS"><div class="titlepage"><div><div><h3 class="title">65.4.1. SP-GiST Limits</h3></div></div></div><p>

  7.    Individual leaf tuples and inner tuples must fit on a single index page

  8.    (8kB by default).  Therefore, when indexing values of variable-length

  9.    data types, long values can only be supported by methods such as radix

  10.    trees, in which each level of the tree includes a prefix that is short

  11.    enough to fit on a page, and the final leaf level includes a suffix also

  12.    short enough to fit on a page.  The operator class should set

  13.    <code class="structfield">longValuesOK</code> to true only if it is prepared to arrange for

  14.    this to happen.  Otherwise, the <acronym class="acronym">SP-GiST</acronym> core will

  15.    reject any request to index a value that is too large to fit

  16.    on an index page.

  17.   </p><p>

  18.    Likewise, it is the operator class's responsibility that inner tuples

  19.    do not grow too large to fit on an index page; this limits the number

  20.    of child nodes that can be used in one inner tuple, as well as the

  21.    maximum size of a prefix value.

  22.   </p><p>

  23.    Another limitation is that when an inner tuple's node points to a set

  24.    of leaf tuples, those tuples must all be in the same index page.

  25.    (This is a design decision to reduce seeking and save space in the

  26.    links that chain such tuples together.)  If the set of leaf tuples

  27.    grows too large for a page, a split is performed and an intermediate

  28.    inner tuple is inserted.  For this to fix the problem, the new inner

  29.    tuple <span class="emphasis"><em>must</em></span> divide the set of leaf values into more than one

  30.    node group.  If the operator class's <code class="function">picksplit</code> function

  31.    fails to do that, the <acronym class="acronym">SP-GiST</acronym> core resorts to

  32.    extraordinary measures described in <a class="xref" href="spgist-implementation.html#SPGIST-ALL-THE-SAME" title="65.4.3. “All-the-Same” Inner Tuples">Section 65.4.3</a>.

  33.   </p></div><div class="sect2" id="SPGIST-NULL-LABELS"><div class="titlepage"><div><div><h3 class="title">65.4.2. SP-GiST Without Node Labels</h3></div></div></div><p>

  34.    Some tree algorithms use a fixed set of nodes for each inner tuple;

  35.    for example, in a quad-tree there are always exactly four nodes

  36.    corresponding to the four quadrants around the inner tuple's centroid

  37.    point.  In such a case the code typically works with the nodes by

  38.    number, and there is no need for explicit node labels.  To suppress

  39.    node labels (and thereby save some space), the <code class="function">picksplit</code>

  40.    function can return NULL for the <code class="structfield">nodeLabels</code> array,

  41.    and likewise the <code class="function">choose</code> function can return NULL for

  42.    the <code class="structfield">prefixNodeLabels</code> array during

  43.    a <code class="literal">spgSplitTuple</code> action.

  44.    This will in turn result in <code class="structfield">nodeLabels</code> being NULL during

  45.    subsequent calls to <code class="function">choose</code> and <code class="function">inner_consistent</code>.

  46.    In principle, node labels could be used for some inner tuples and omitted

  47.    for others in the same index.

  48.   </p><p>

  49.    When working with an inner tuple having unlabeled nodes, it is an error

  50.    for <code class="function">choose</code> to return <code class="literal">spgAddNode</code>, since the set

  51.    of nodes is supposed to be fixed in such cases.

  52.   </p></div><div class="sect2" id="SPGIST-ALL-THE-SAME"><div class="titlepage"><div><div><h3 class="title">65.4.3. <span class="quote">“<span class="quote">All-the-Same</span>”</span> Inner Tuples</h3></div></div></div><p>

  53.    The <acronym class="acronym">SP-GiST</acronym> core can override the results of the

  54.    operator class's <code class="function">picksplit</code> function when

  55.    <code class="function">picksplit</code> fails to divide the supplied leaf values into

  56.    at least two node categories.  When this happens, the new inner tuple

  57.    is created with multiple nodes that each have the same label (if any)

  58.    that <code class="function">picksplit</code> gave to the one node it did use, and the

  59.    leaf values are divided at random among these equivalent nodes.

  60.    The <code class="literal">allTheSame</code> flag is set on the inner tuple to warn the

  61.    <code class="function">choose</code> and <code class="function">inner_consistent</code> functions that the

  62.    tuple does not have the node set that they might otherwise expect.

  63.   </p><p>

  64.    When dealing with an <code class="literal">allTheSame</code> tuple, a <code class="function">choose</code>

  65.    result of <code class="literal">spgMatchNode</code> is interpreted to mean that the new

  66.    value can be assigned to any of the equivalent nodes; the core code will

  67.    ignore the supplied  <code class="structfield">nodeN</code> value and descend into one

  68.    of the nodes at random (so as to keep the tree balanced).  It is an

  69.    error for <code class="function">choose</code> to return <code class="literal">spgAddNode</code>, since

  70.    that would make the nodes not all equivalent; the

  71.    <code class="literal">spgSplitTuple</code> action must be used if the value to be inserted

  72.    doesn't match the existing nodes.

  73.   </p><p>

  74.    When dealing with an <code class="literal">allTheSame</code> tuple, the

  75.    <code class="function">inner_consistent</code> function should return either all or none

  76.    of the nodes as targets for continuing the index search, since they are

  77.    all equivalent.  This may or may not require any special-case code,

  78.    depending on how much the <code class="function">inner_consistent</code> function normally

  79.    assumes about the meaning of the nodes.

  80.   </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="spgist-extensibility.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="spgist.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="spgist-examples.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">65.3. Extensibility </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 65.5. Examples</td></tr></table></div></body></html>
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