diff options
Diffstat (limited to 'doc/src/sgml/ref/create_operator.sgml')
| -rw-r--r-- | doc/src/sgml/ref/create_operator.sgml | 87 |
1 files changed, 60 insertions, 27 deletions
diff --git a/doc/src/sgml/ref/create_operator.sgml b/doc/src/sgml/ref/create_operator.sgml index 7f06c4cece3..36d791d2a79 100644 --- a/doc/src/sgml/ref/create_operator.sgml +++ b/doc/src/sgml/ref/create_operator.sgml @@ -1,5 +1,5 @@ <!-- -$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_operator.sgml,v 1.11 1999/07/22 15:09:08 thomas Exp $ +$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_operator.sgml,v 1.12 2000/03/18 18:03:12 tgl Exp $ Postgres documentation --> @@ -60,8 +60,8 @@ CREATE OPERATOR <replaceable>name</replaceable> ( PROCEDURE = <replaceable class <term><replaceable class="parameter">type1</replaceable></term> <listitem> <para> - The type for the left-hand side of the operator, if any. This option would be - omitted for a right-unary operator. + The type of the left-hand argument of the operator, if any. + This option would be omitted for a left-unary operator. </para> </listitem> </varlistentry> @@ -69,8 +69,8 @@ CREATE OPERATOR <replaceable>name</replaceable> ( PROCEDURE = <replaceable class <term><replaceable class="parameter">type2</replaceable></term> <listitem> <para> - The type for the right-hand side of the operator, if any. This option would be - omitted for a left-unary operator. + The type of the right-hand argument of the operator, if any. + This option would be omitted for a right-unary operator. </para> </listitem> </varlistentry> @@ -78,7 +78,7 @@ CREATE OPERATOR <replaceable>name</replaceable> ( PROCEDURE = <replaceable class <term><replaceable class="parameter">com_op</replaceable></term> <listitem> <para> - The commutator for this operator. + The commutator of this operator. </para> </listitem> </varlistentry> @@ -110,7 +110,7 @@ CREATE OPERATOR <replaceable>name</replaceable> ( PROCEDURE = <replaceable class <term>HASHES</term> <listitem> <para> -Indicates this operator can support a hash-join algorithm. + Indicates this operator can support a hash join. </para> </listitem> </varlistentry> @@ -118,7 +118,8 @@ Indicates this operator can support a hash-join algorithm. <term><replaceable class="parameter">left_sort_op</replaceable></term> <listitem> <para> - Operator that sorts the left-hand data type of this operator. + If this operator can support a merge join, the + operator that sorts the left-hand data type of this operator. </para> </listitem> </varlistentry> @@ -126,7 +127,8 @@ Indicates this operator can support a hash-join algorithm. <term><replaceable class="parameter">right_sort_op</replaceable></term> <listitem> <para> - Operator that sorts the right-hand data type of this operator. + If this operator can support a merge join, the + operator that sorts the right-hand data type of this operator. </para> </listitem> </varlistentry> @@ -172,22 +174,56 @@ CREATE </para> <para> The operator <replaceable class="parameter">name</replaceable> - is a sequence of up to thirty two (32) characters in any combination - from the following: + is a sequence of up to NAMEDATALEN-1 (31 by default) characters + from the following list: <literallayout> -+ - * / < > = ~ ! @ # % ^ & | ` ? $ : ++ - * / < > = ~ ! @ # % ^ & | ` ? $ : </literallayout> + + There are a few restrictions on your choice of name: + <itemizedlist> + <listitem> + <para> + "$" and ":" cannot be defined as single-character operators, + although they can be part of a multi-character operator name. + </para> + </listitem> + <listitem> + <para> + "--" and "/*" cannot appear anywhere in an operator name, + since they will be taken as the start of a comment. + </para> + </listitem> + <listitem> + <para> + A multi-character operator name cannot end in "+" or "-", + unless the name also contains at least one of these characters: + <literallayout> +~ ! @ # % ^ & | ` ? $ : + </literallayout> + For example, <literal>@-</literal> is an allowed operator name, + but <literal>*-</literal> is not. + This restriction allows <productname>Postgres</productname> to + parse SQL-compliant queries without requiring spaces between tokens. + </para> + </listitem> + </itemizedlist> + <note> <para> - No alphabetic characters are allowed in an operator name. - This enables <productname>Postgres</productname> to parse SQL input - into tokens without requiring spaces between each token. + When working with non-SQL-standard operator names, you will usually + need to separate adjacent operators with spaces to avoid ambiguity. + For example, if you have defined a left-unary operator named "@", + you cannot write <literal>X*@Y</literal>; you must write + <literal>X* @Y</literal> to ensure that + <productname>Postgres</productname> reads it as two operator names + not one. </para> </note> </para> <para> - The operator "!=" is mapped to "<>" on input, so they are - therefore equivalent. + The operator "!=" is mapped to "<>" on input, so these two names + are always equivalent. </para> <para> At least one of LEFTARG and RIGHTARG must be defined. For @@ -196,11 +232,11 @@ CREATE unary operators only RIGHTARG should be defined. </para> <para> - Also, the + The <replaceable class="parameter">func_name</replaceable> procedure must have been previously defined using <command>CREATE FUNCTION</command> and must be defined to accept the correct number of arguments - (either one or two). + (either one or two) of the indicated types. </para> <para> The commutator operator should be identified if one exists, @@ -247,8 +283,6 @@ MYBOXES.description !== "0,0,1,1"::box does not yet have a commutator itself, then the commutator's entry is updated to have the newly created operator as its commutator. This applies to the negator, as well. - </para> - <para> This is to allow the definition of two operators that are the commutators or the negators of each other. The first operator should be defined without a commutator or negator @@ -258,7 +292,7 @@ MYBOXES.description !== "0,0,1,1"::box it also works to just have both operators refer to each other.) </para> <para> - The next three specifications are present to support the + The HASHES, SORT1, and SORT2 options are present to support the query optimizer in performing joins. <productname>Postgres</productname> can always evaluate a join (i.e., processing a clause with two tuple @@ -294,9 +328,8 @@ MYBOXES.description !== "0,0,1,1"::box be worth the complexity involved. </para> <para> - The last two pieces of the specification are present so - the query optimizer can estimate result sizes. If a - clause of the form: + The RESTRICT and JOIN options assist the query optimizer in estimating + result sizes. If a clause of the form: <programlisting> MYBOXES.description <<< "0,0,1,1"::box </programlisting> @@ -310,7 +343,7 @@ MYBOXES.description <<< "0,0,1,1"::box data types and returns a floating point number. The query optimizer simply calls this function, passing the parameter "0,0,1,1" and multiplies the result by the relation - size to get the desired expected number of instances. + size to get the expected number of instances. </para> <para> Similarly, when the operands of the operator both contain @@ -318,7 +351,7 @@ MYBOXES.description <<< "0,0,1,1"::box size of the resulting join. The function join_proc will return another floating point number which will be multiplied by the cardinalities of the two classes involved to - compute the desired expected result size. + compute the expected result size. </para> <para> The difference between the function |