xoper.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

    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 <function>eqsel</function> on the assumption that
    they'll usually only match a small fraction of the entries in a table.
   </para>

   <para>
    You can use <function>scalarltsel</> and <function>scalargtsel</> 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 <function>convert_to_scalar()</function> in <filename>src/backend/utils/adt/selfuncs.c</filename>.
    (Eventually, this function should be replaced by per-data-type functions
    identified through a column of the <classname>pg_type</> 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.
   </para>

   <para>
    There are additional selectivity estimation functions designed for geometric
    operators in <filename>src/backend/utils/adt/geo_selfuncs.c</filename>: <function>areasel</function>, <function>positionsel</function>,
    and <function>contsel</function>.  At this writing these are just stubs, but you may want
    to use them (or even better, improve them) anyway.
   </para>
   </sect2>

   <sect2>
    <title><literal>JOIN</></title>

    <para>
     The <literal>JOIN</> clause, if provided, names a join selectivity
     estimation function for the operator.  (Note that this is a function
     name, not an operator name.)  <literal>JOIN</> clauses only make sense for
     binary operators that return <type>boolean</type>.  The idea behind a join
     selectivity estimator is to guess what fraction of the rows in a
     pair of tables will satisfy a <literal>WHERE</>-clause condition of the form
<programlisting>
table1.column1 OP table2.column2
</programlisting>
     for the current operator.  As with the <literal>RESTRICT</literal> 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.
    </para>

    <para>
     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:
     <simplelist>
      <member><function>eqjoinsel</> for <literal>=</></member>
      <member><function>neqjoinsel</> for <literal>&lt;&gt;</></member>
      <member><function>scalarltjoinsel</> for <literal>&lt;</> or <literal>&lt;=</></member>
      <member><function>scalargtjoinsel</> for <literal>&gt;</> or <literal>&gt;=</></member>
      <member><function>areajoinsel</> for 2D area-based comparisons</member>
      <member><function>positionjoinsel</> for 2D position-based comparisons</member>
      <member><function>contjoinsel</> for 2D containment-based comparisons</member>
     </simplelist>
    </para>
   </sect2>

   <sect2>
    <title><literal>HASHES</></title>

    <para>
     The <literal>HASHES</literal> clause, if present, tells the system that
     it is permissible to use the hash join method for a join based on this
     operator.  <literal>HASHES</> only makes sense for a binary operator that
     returns <literal>boolean</>, and in practice the operator had better be
     equality for some data type.
    </para>

    <para>
     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 <literal>HASHES</literal> for operators that do not represent
     equality.
    </para>

    <para>
     To be marked <literal>HASHES</literal>, the join operator must appear
     in a hash index operator class.  This is not enforced when you create
     the operator, since of course the referencing operator class couldn't
     exist yet.  But attempts to use the operator in hash joins will fail
     at run time if no such operator class exists.  The system needs the
     operator class to find the data-type-specific hash function for the
     operator's input data type.  Of course, you must also supply a suitable
     hash function before you can create the operator class.
    </para>

    <para>
     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 may be
     uninteresting pad bits, you can't simply pass the whole structure to
     <function>hash_any</>.  (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>IEEE</>
     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.
    </para>

    <note>
    <para>
     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.
    </para>
    </note>

    <note>
    <para>
     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 <literal>IN</> operations may
     generate wrong results.  (Specifically, <literal>IN</> 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.)
    </para>
    </note>

   </sect2>

   <sect2>
    <title><literal>MERGES</> (<literal>SORT1</>, <literal>SORT2</>, <literal>LTCMP</>, <literal>GTCMP</>)</title>

    <para>
     The <literal>MERGES</literal> clause, if present, tells the system that
     it is permissible to use the merge-join method for a join based on this
     operator.  <literal>MERGES</> only makes sense for a binary operator that
     returns <literal>boolean</>, and in practice the operator must represent
     equality for some data type or pair of data types.
    </para>

    <para>
     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
     <quote>same place</>
     in the sort order.  In practice this means that the join operator must
     behave like equality.  But unlike hash join, where the left and right
     data types had better be the same (or at least bitwise equivalent),
     it is possible to merge-join two
     distinct data types so long as they are logically compatible.  For
     example, the <type>smallint</type>-versus-<type>integer</type> equality operator
     is merge-joinable.
     We only need sorting operators that will bring both data types into a
     logically compatible sequence.
    </para>

    <para>
     Execution of a merge join requires that the system be able to identify
     four operators related to the merge-join equality operator: less-than
     comparison for the left operand data type, less-than comparison for the
     right operand data type, less-than comparison between the two data types, and
     greater-than comparison between the two data types.  (These are actually
     four distinct operators if the merge-joinable operator has two different
     operand data types; but when the operand types are the same the three
     less-than operators are all the same operator.)
     It is possible to
     specify these operators individually by name, as the <literal>SORT1</>,
     <literal>SORT2</>, <literal>LTCMP</>, and <literal>GTCMP</> options
     respectively.  The system will fill in the default names
     <literal>&lt;</>, <literal>&lt;</>, <literal>&lt;</>, <literal>&gt;</>
     respectively if any of these are omitted when <literal>MERGES</> is
     specified.  Also, <literal>MERGES</> will be assumed to be implied if any
     of these four operator options appear, so it is possible to specify
     just some of them and let the system fill in the rest.
    </para>

    <para>
     The operand data types of the four comparison operators can be deduced
     from the operand types of the merge-joinable operator, so just as with
     <literal>COMMUTATOR</>, only the operator names need be given in these
     clauses.  Unless you are using peculiar choices of operator names,
     it's sufficient to write <literal>MERGES</> and let the system fill in
     the details.
     (As with <literal>COMMUTATOR</> and <literal>NEGATOR</>, the system is
     able to make dummy
     operator entries if you happen to define the equality operator before
     the other ones.)
    </para>

    <para>
     There are additional restrictions on operators that you mark
     merge-joinable.  These restrictions are not currently checked by
     <command>CREATE OPERATOR</command>, but errors may occur when
     the operator is used if any are not true:

     <itemizedlist>
      <listitem>
       <para>
	A merge-joinable equality operator must have a merge-joinable
        commutator (itself if the two operand data types are the same, or a related
        equality operator if they are different).
       </para>
      </listitem>

      <listitem>
       <para>
        If there is a merge-joinable operator relating any two data types
	A and B, and another merge-joinable operator relating B to any
	third data type C, then A and C must also have a merge-joinable
	operator; in other words, having a merge-joinable operator must
	be transitive.
       </para>
      </listitem>

      <listitem>
       <para>
        Bizarre results will ensue at run time if the four comparison
	operators you name do not sort the data values compatibly.
       </para>
      </listitem>
     </itemizedlist>
    </para>

    <note>
    <para>
     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.
    </para>
    </note>

    <note>
    <para>
     In <productname>PostgreSQL</productname> versions before 7.3,
     the <literal>MERGES</> shorthand was not available: to make a
     merge-joinable operator one had to write both <literal>SORT1</> and
     <literal>SORT2</> explicitly.  Also, the <literal>LTCMP</> and
     <literal>GTCMP</>
     options did not exist; the names of those operators were hardwired as
     <literal>&lt;</> and <literal>&gt;</> respectively.
    </para>
    </note>
   </sect2>
  </sect1>

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