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<!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>37.15. Operator Optimization Information</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="xoper.html" title="37.14. User-Defined Operators" /><link rel="next" href="xindex.html" title="37.16. Interfacing Extensions to Indexes" /></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">37.15. Operator Optimization Information</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="xoper.html" title="37.14. User-Defined Operators">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="extend.html" title="Chapter 37. Extending SQL">Up</a></td><th width="60%" align="center">Chapter 37. Extending <acronym xmlns="http://www.w3.org/1999/xhtml" class="acronym">SQL</acronym></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="xindex.html" title="37.16. Interfacing Extensions to Indexes">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="XOPER-OPTIMIZATION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">37.15. Operator Optimization Information</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="xoper-optimization.html#id-1.8.3.18.6">37.15.1. <code class="literal">COMMUTATOR</code></a></span></dt><dt><span class="sect2"><a href="xoper-optimization.html#id-1.8.3.18.7">37.15.2. <code class="literal">NEGATOR</code></a></span></dt><dt><span class="sect2"><a href="xoper-optimization.html#id-1.8.3.18.8">37.15.3. <code class="literal">RESTRICT</code></a></span></dt><dt><span class="sect2"><a href="xoper-optimization.html#id-1.8.3.18.9">37.15.4. <code class="literal">JOIN</code></a></span></dt><dt><span class="sect2"><a href="xoper-optimization.html#id-1.8.3.18.10">37.15.5. <code class="literal">HASHES</code></a></span></dt><dt><span class="sect2"><a href="xoper-optimization.html#id-1.8.3.18.11">37.15.6. <code class="literal">MERGES</code></a></span></dt></dl></div><a id="id-1.8.3.18.2" class="indexterm"></a><p>
    A <span class="productname">PostgreSQL</span> operator definition can include
    several optional clauses that tell the system useful things about how
    the operator behaves.  These clauses should be provided whenever
    appropriate, because they can make for considerable speedups in execution
    of queries that use the operator.  But if you provide them, you must be
    sure that they are right!  Incorrect use of an optimization clause can
    result in slow queries, subtly wrong output, or other Bad Things.
    You can always leave out an optimization clause if you are not sure
    about it; the only consequence is that queries might run slower than
    they need to.
   </p><p>
    Additional optimization clauses might be added in future versions of
    <span class="productname">PostgreSQL</span>.  The ones described here are all
    the ones that release 12.4 understands.
   </p><p>
    It is also possible to attach a planner support function to the function
    that underlies an operator, providing another way of telling the system
    about the behavior of the operator.
    See <a class="xref" href="xfunc-optimization.html" title="37.11. Function Optimization Information">Section 37.11</a> for more information.
   </p><div class="sect2" id="id-1.8.3.18.6"><div class="titlepage"><div><div><h3 class="title">37.15.1. <code class="literal">COMMUTATOR</code></h3></div></div></div><p>
     The <code class="literal">COMMUTATOR</code> clause, if provided, names an operator that is the
     commutator of the operator being defined.  We say that operator A is the
     commutator of operator B if (x A y) equals (y B x) for all possible input
     values x, y.  Notice that B is also the commutator of A.  For example,
     operators <code class="literal">&lt;</code> and <code class="literal">&gt;</code> for a particular data type are usually each others'
     commutators, and operator <code class="literal">+</code> is usually commutative with itself.
     But operator <code class="literal">-</code> is usually not commutative with anything.
    </p><p>
     The left operand type of a commutable operator is the same as the
     right operand type of its commutator, and vice versa.  So the name of
     the commutator operator is all that <span class="productname">PostgreSQL</span>
     needs to be given to look up the commutator, and that's all that needs to
     be provided in the <code class="literal">COMMUTATOR</code> clause.
    </p><p>
     It's critical to provide commutator information for operators that
     will be used in indexes and join clauses, because this allows the
     query optimizer to <span class="quote">“<span class="quote">flip around</span>”</span> such a clause to the forms
     needed for different plan types.  For example, consider a query with
     a WHERE clause like <code class="literal">tab1.x = tab2.y</code>, where <code class="literal">tab1.x</code>
     and <code class="literal">tab2.y</code> are of a user-defined type, and suppose that
     <code class="literal">tab2.y</code> is indexed.  The optimizer cannot generate an
     index scan unless it can determine how to flip the clause around to
     <code class="literal">tab2.y = tab1.x</code>, because the index-scan machinery expects
     to see the indexed column on the left of the operator it is given.
     <span class="productname">PostgreSQL</span> will <span class="emphasis"><em>not</em></span> simply
     assume that this is a valid transformation — the creator of the
     <code class="literal">=</code> operator must specify that it is valid, by marking the
     operator with commutator information.
    </p><p>
     When you are defining a self-commutative operator, you just do it.
     When you are defining a pair of commutative operators, things are
     a little trickier: how can the first one to be defined refer to the
     other one, which you haven't defined yet?  There are two solutions
     to this problem:

     </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
        One way is to omit the <code class="literal">COMMUTATOR</code> clause in the first operator that
        you define, and then provide one in the second operator's definition.
        Since <span class="productname">PostgreSQL</span> knows that commutative
        operators come in pairs, when it sees the second definition it will
        automatically go back and fill in the missing <code class="literal">COMMUTATOR</code> clause in
        the first definition.
       </p></li><li class="listitem"><p>
        The other, more straightforward way is just to include <code class="literal">COMMUTATOR</code> clauses
        in both definitions.  When <span class="productname">PostgreSQL</span> processes
        the first definition and realizes that <code class="literal">COMMUTATOR</code> refers to a nonexistent
        operator, the system will make a dummy entry for that operator in the
        system catalog.  This dummy entry will have valid data only
        for the operator name, left and right operand types, and result type,
        since that's all that <span class="productname">PostgreSQL</span> can deduce
        at this point.  The first operator's catalog entry will link to this
        dummy entry.  Later, when you define the second operator, the system
        updates the dummy entry with the additional information from the second
        definition.  If you try to use the dummy operator before it's been filled
        in, you'll just get an error message.
       </p></li></ul></div><p>
    </p></div><div class="sect2" id="id-1.8.3.18.7"><div class="titlepage"><div><div><h3 class="title">37.15.2. <code class="literal">NEGATOR</code></h3></div></div></div><p>
     The <code class="literal">NEGATOR</code> clause, if provided, names an operator that is the
     negator of the operator being defined.  We say that operator A
     is the negator of operator B if both return Boolean results and
     (x A y) equals NOT (x B y) for all possible inputs x, y.
     Notice that B is also the negator of A.
     For example, <code class="literal">&lt;</code> and <code class="literal">&gt;=</code> are a negator pair for most data types.
     An operator can never validly be its own negator.
    </p><p>
    Unlike commutators, a pair of unary operators could validly be marked
    as each other's negators; that would mean (A x) equals NOT (B x)
    for all x, or the equivalent for right unary operators.
   </p><p>
    An operator's negator must have the same left and/or right operand types
    as the operator to be defined, so just as with <code class="literal">COMMUTATOR</code>, only the operator
    name need be given in the <code class="literal">NEGATOR</code> clause.
   </p><p>
    Providing a negator is very helpful to the query optimizer since
    it allows expressions like <code class="literal">NOT (x = y)</code> to be simplified into
    <code class="literal">x &lt;&gt; y</code>.  This comes up more often than you might think, because
    <code class="literal">NOT</code> operations can be inserted as a consequence of other rearrangements.
   </p><p>
    Pairs of negator operators can be defined using the same methods
    explained above for commutator pairs.
   </p></div><div class="sect2" id="id-1.8.3.18.8"><div class="titlepage"><div><div><h3 class="title">37.15.3. <code class="literal">RESTRICT</code></h3></div></div></div><p>
    The <code class="literal">RESTRICT</code> clause, if provided, names a restriction selectivity
    estimation function for the operator.  (Note that this is a function
    name, not an operator name.)  <code class="literal">RESTRICT</code> clauses only make sense for
    binary operators that return <code class="type">boolean</code>.  The idea behind a restriction
    selectivity estimator is to guess what fraction of the rows in a
    table will satisfy a <code class="literal">WHERE</code>-clause condition of the form:
</p><pre class="programlisting">
column OP constant
</pre><p>
    for the current operator and a particular constant value.
    This assists the optimizer by
    giving it some idea of how many rows will be eliminated by <code class="literal">WHERE</code>
    clauses that have this form.  (What happens if the constant is on
    the left, you might be wondering?  Well, that's one of the things that
    <code class="literal">COMMUTATOR</code> is for...)
   </p><p>
    Writing new restriction selectivity estimation functions is far beyond
    the scope of this chapter, but fortunately you can usually just use
    one of the system's standard estimators for many of your own operators.
    These are the standard restriction estimators:
    </p><table border="0" summary="Simple list" class="simplelist"><tr><td><code class="function">eqsel</code> for <code class="literal">=</code></td></tr><tr><td><code class="function">neqsel</code> for <code class="literal">&lt;&gt;</code></td></tr><tr><td><code class="function">scalarltsel</code> for <code class="literal">&lt;</code></td></tr><tr><td><code class="function">scalarlesel</code> for <code class="literal">&lt;=</code></td></tr><tr><td><code class="function">scalargtsel</code> for <code class="literal">&gt;</code></td></tr><tr><td><code class="function">scalargesel</code> for <code class="literal">&gt;=</code></td></tr></table><p>
   </p><p>
    You can frequently get away with using either <code class="function">eqsel</code> or <code class="function">neqsel</code> for
    operators that have very high or very low selectivity, even if they
    aren't really equality or inequality.  For example, the
    approximate-equality geometric operators use <code class="function">eqsel</code> on the assumption that
    they'll usually only match a small fraction of the entries in a table.
   </p><p>
    You can use <code class="function">scalarltsel</code>, <code class="function">scalarlesel</code>,
    <code class="function">scalargtsel</code> and <code class="function">scalargesel</code> for comparisons on
    data types that have some sensible means of being converted into numeric
    scalars for range comparisons.  If possible, add the data type to those
    understood by the function <code class="function">convert_to_scalar()</code> in
    <code class="filename">src/backend/utils/adt/selfuncs.c</code>.
    (Eventually, this function should be replaced by per-data-type functions
    identified through a column of the <code class="classname">pg_type</code> system catalog; but that hasn't happened
    yet.)  If you do not do this, things will still work, but the optimizer's
    estimates won't be as good as they could be.
   </p><p>
    There are additional selectivity estimation functions designed for geometric
    operators in <code class="filename">src/backend/utils/adt/geo_selfuncs.c</code>: <code class="function">areasel</code>, <code class="function">positionsel</code>,
    and <code class="function">contsel</code>.  At this writing these are just stubs, but you might want
    to use them (or even better, improve them) anyway.
   </p></div><div class="sect2" id="id-1.8.3.18.9"><div class="titlepage"><div><div><h3 class="title">37.15.4. <code class="literal">JOIN</code></h3></div></div></div><p>
     The <code class="literal">JOIN</code> clause, if provided, names a join selectivity
     estimation function for the operator.  (Note that this is a function
     name, not an operator name.)  <code class="literal">JOIN</code> clauses only make sense for
     binary operators that return <code class="type">boolean</code>.  The idea behind a join
     selectivity estimator is to guess what fraction of the rows in a
     pair of tables will satisfy a <code class="literal">WHERE</code>-clause condition of the form:
</p><pre class="programlisting">
table1.column1 OP table2.column2
</pre><p>
     for the current operator.  As with the <code class="literal">RESTRICT</code> clause, this helps
     the optimizer very substantially by letting it figure out which
     of several possible join sequences is likely to take the least work.
    </p><p>
     As before, this chapter will make no attempt to explain how to write
     a join selectivity estimator function, but will just suggest that
     you use one of the standard estimators if one is applicable:
     </p><table border="0" summary="Simple list" class="simplelist"><tr><td><code class="function">eqjoinsel</code> for <code class="literal">=</code></td></tr><tr><td><code class="function">neqjoinsel</code> for <code class="literal">&lt;&gt;</code></td></tr><tr><td><code class="function">scalarltjoinsel</code> for <code class="literal">&lt;</code></td></tr><tr><td><code class="function">scalarlejoinsel</code> for <code class="literal">&lt;=</code></td></tr><tr><td><code class="function">scalargtjoinsel</code> for <code class="literal">&gt;</code></td></tr><tr><td><code class="function">scalargejoinsel</code> for <code class="literal">&gt;=</code></td></tr><tr><td><code class="function">areajoinsel</code> for 2D area-based comparisons</td></tr><tr><td><code class="function">positionjoinsel</code> for 2D position-based comparisons</td></tr><tr><td><code class="function">contjoinsel</code> for 2D containment-based comparisons</td></tr></table><p>
    </p></div><div class="sect2" id="id-1.8.3.18.10"><div class="titlepage"><div><div><h3 class="title">37.15.5. <code class="literal">HASHES</code></h3></div></div></div><p>
     The <code class="literal">HASHES</code> clause, if present, tells the system that
     it is permissible to use the hash join method for a join based on this
     operator.  <code class="literal">HASHES</code> only makes sense for a binary operator that
     returns <code class="literal">boolean</code>, and in practice the operator must represent
     equality for some data type or pair of data types.
    </p><p>
     The assumption underlying hash join is that the join operator can
     only return true for pairs of left and right values that hash to the
     same hash code.  If two values get put in different hash buckets, the
     join will never compare them at all, implicitly assuming that the
     result of the join operator must be false.  So it never makes sense
     to specify <code class="literal">HASHES</code> for operators that do not represent
     some form of equality.  In most cases it is only practical to support
     hashing for operators that take the same data type on both sides.
     However, sometimes it is possible to design compatible hash functions
     for two or more data types; that is, functions that will generate the
     same hash codes for <span class="quote">“<span class="quote">equal</span>”</span> values, even though the values
     have different representations.  For example, it's fairly simple
     to arrange this property when hashing integers of different widths.
    </p><p>
     To be marked <code class="literal">HASHES</code>, the join operator must appear
     in a hash index operator family.  This is not enforced when you create
     the operator, since of course the referencing operator family couldn't
     exist yet.  But attempts to use the operator in hash joins will fail
     at run time if no such operator family exists.  The system needs the
     operator family to find the data-type-specific hash function(s) for the
     operator's input data type(s).  Of course, you must also create suitable
     hash functions before you can create the operator family.
    </p><p>
     Care should be exercised when preparing a hash function, because there
     are machine-dependent ways in which it might fail to do the right thing.
     For example, if your data type is a structure in which there might be
     uninteresting pad bits, you cannot simply pass the whole structure to
     <code class="function">hash_any</code>.  (Unless you write your other operators and
     functions to ensure that the unused bits are always zero, which is the
     recommended strategy.)
     Another example is that on machines that meet the <acronym class="acronym">IEEE</acronym>
     floating-point standard, negative zero and positive zero are different
     values (different bit patterns) but they are defined to compare equal.
     If a float value might contain negative zero then extra steps are needed
     to ensure it generates the same hash value as positive zero.
    </p><p>
     A hash-joinable operator must have a commutator (itself if the two
     operand data types are the same, or a related equality operator
     if they are different) that appears in the same operator family.
     If this is not the case, planner errors might occur when the operator
     is used.  Also, it is a good idea (but not strictly required) for
     a hash operator family that supports multiple data types to provide
     equality operators for every combination of the data types; this
     allows better optimization.
    </p><div class="note"><h3 class="title">Note</h3><p>
     The function underlying a hash-joinable operator must be marked
     immutable or stable.  If it is volatile, the system will never
     attempt to use the operator for a hash join.
    </p></div><div class="note"><h3 class="title">Note</h3><p>
     If a hash-joinable operator has an underlying function that is marked
     strict, the
     function must also be complete: that is, it should return true or
     false, never null, for any two nonnull inputs.  If this rule is
     not followed, hash-optimization of <code class="literal">IN</code> operations might
     generate wrong results.  (Specifically, <code class="literal">IN</code> might return
     false where the correct answer according to the standard would be null;
     or it might yield an error complaining that it wasn't prepared for a
     null result.)
    </p></div></div><div class="sect2" id="id-1.8.3.18.11"><div class="titlepage"><div><div><h3 class="title">37.15.6. <code class="literal">MERGES</code></h3></div></div></div><p>
     The <code class="literal">MERGES</code> clause, if present, tells the system that
     it is permissible to use the merge-join method for a join based on this
     operator.  <code class="literal">MERGES</code> only makes sense for a binary operator that
     returns <code class="literal">boolean</code>, and in practice the operator must represent
     equality for some data type or pair of data types.
    </p><p>
     Merge join is based on the idea of sorting the left- and right-hand tables
     into order and then scanning them in parallel.  So, both data types must
     be capable of being fully ordered, and the join operator must be one
     that can only succeed for pairs of values that fall at the
     <span class="quote">“<span class="quote">same place</span>”</span>
     in the sort order.  In practice this means that the join operator must
     behave like equality.  But it is possible to merge-join two
     distinct data types so long as they are logically compatible.  For
     example, the <code class="type">smallint</code>-versus-<code class="type">integer</code>
     equality operator is merge-joinable.
     We only need sorting operators that will bring both data types into a
     logically compatible sequence.
    </p><p>
     To be marked <code class="literal">MERGES</code>, the join operator must appear
     as an equality member of a <code class="literal">btree</code> index operator family.
     This is not enforced when you create
     the operator, since of course the referencing operator family couldn't
     exist yet.  But the operator will not actually be used for merge joins
     unless a matching operator family can be found.  The
     <code class="literal">MERGES</code> flag thus acts as a hint to the planner that
     it's worth looking for a matching operator family.
    </p><p>
     A merge-joinable operator must have a commutator (itself if the two
     operand data types are the same, or a related equality operator
     if they are different) that appears in the same operator family.
     If this is not the case, planner errors might occur when the operator
     is used.  Also, it is a good idea (but not strictly required) for
     a <code class="literal">btree</code> operator family that supports multiple data types to provide
     equality operators for every combination of the data types; this
     allows better optimization.
    </p><div class="note"><h3 class="title">Note</h3><p>
     The function underlying a merge-joinable operator must be marked
     immutable or stable.  If it is volatile, the system will never
     attempt to use the operator for a merge join.
    </p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="xoper.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="extend.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="xindex.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">37.14. User-Defined Operators </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 37.16. Interfacing Extensions to Indexes</td></tr></table></div></body></html>