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diff --git a/doc/src/sgml/html/xoper-optimization.html b/doc/src/sgml/html/xoper-optimization.html new file mode 100644 index 0000000..5d13846 --- /dev/null +++ b/doc/src/sgml/html/xoper-optimization.html @@ -0,0 +1,281 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!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>38.15. 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 Vsnapshot" /><link rel="prev" href="xoper.html" title="38.14. User-Defined Operators" /><link rel="next" href="xindex.html" title="38.16. Interfacing Extensions to Indexes" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">38.15. Operator Optimization Information</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="xoper.html" title="38.14. User-Defined Operators">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="extend.html" title="Chapter 38. Extending SQL">Up</a></td><th width="60%" align="center">Chapter 38. Extending <acronym class="acronym">SQL</acronym></th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="xindex.html" title="38.16. Interfacing Extensions to Indexes">Next</a></td></tr></table><hr /></div><div class="sect1" id="XOPER-OPTIMIZATION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">38.15. 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">38.15.1. <code class="literal">COMMUTATOR</code></a></span></dt><dt><span class="sect2"><a href="xoper-optimization.html#id-1.8.3.18.7">38.15.2. <code class="literal">NEGATOR</code></a></span></dt><dt><span class="sect2"><a href="xoper-optimization.html#id-1.8.3.18.8">38.15.3. <code class="literal">RESTRICT</code></a></span></dt><dt><span class="sect2"><a href="xoper-optimization.html#id-1.8.3.18.9">38.15.4. <code class="literal">JOIN</code></a></span></dt><dt><span class="sect2"><a href="xoper-optimization.html#id-1.8.3.18.10">38.15.5. <code class="literal">HASHES</code></a></span></dt><dt><span class="sect2"><a href="xoper-optimization.html#id-1.8.3.18.11">38.15.6. <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 15.5 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="38.11. Function Optimization Information">Section 38.11</a> for more information. + </p><div class="sect2" id="id-1.8.3.18.6"><div class="titlepage"><div><div><h3 class="title">38.15.1. <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"><</code> and <code class="literal">></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">38.15.2. <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"><</code> and <code class="literal">>=</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. + </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 <> 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">38.15.3. <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"><></code></td></tr><tr><td><code class="function">scalarltsel</code> for <code class="literal"><</code></td></tr><tr><td><code class="function">scalarlesel</code> for <code class="literal"><=</code></td></tr><tr><td><code class="function">scalargtsel</code> for <code class="literal">></code></td></tr><tr><td><code class="function">scalargesel</code> for <code class="literal">>=</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> + Another useful built-in selectivity estimation function + is <code class="function">matchingsel</code>, which will work for almost any + binary operator, if standard MCV and/or histogram statistics are + collected for the input data type(s). Its default estimate is set to + twice the default estimate used in <code class="function">eqsel</code>, making + it most suitable for comparison operators that are somewhat less + strict than equality. (Or you could call the + underlying <code class="function">generic_restriction_selectivity</code> + function, providing a different default estimate.) + </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">38.15.4. <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"><></code></td></tr><tr><td><code class="function">scalarltjoinsel</code> for <code class="literal"><</code></td></tr><tr><td><code class="function">scalarlejoinsel</code> for <code class="literal"><=</code></td></tr><tr><td><code class="function">scalargtjoinsel</code> for <code class="literal">></code></td></tr><tr><td><code class="function">scalargejoinsel</code> for <code class="literal">>=</code></td></tr><tr><td><code class="function">matchingjoinsel</code> for generic matching operators</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">38.15.5. <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">38.15.6. <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" title="38.14. User-Defined Operators">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="extend.html" title="Chapter 38. Extending SQL">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="xindex.html" title="38.16. Interfacing Extensions to Indexes">Next</a></td></tr><tr><td width="40%" align="left" valign="top">38.14. User-Defined Operators </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 38.16. Interfacing Extensions to Indexes</td></tr></table></div></body></html>
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