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提交 bf886d5b 编写于 作者: P Peter Eisentraut
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Add section explaining unspecified expression evaluation order.

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doc/src/sgml/syntax.sgml
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<!--
$Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.60 2002/04/25 20:14:43 tgl Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.61 2002/06/01 20:56:55 petere Exp $
-->
<chapter id="sql-syntax">
......@@ -44,7 +44,6 @@ $Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.60 2002/04/25 20:14:43 tgl
whitespace.
</para>
<informalexample id="sql-syntax-ex-commands">
<para>
For example, the following is (syntactically) valid SQL input:
<programlisting>
......@@ -56,7 +55,6 @@ INSERT INTO MY_TABLE VALUES (3, 'hi there');
is not required; more than one command can be on a line, and
commands can usefully be split across lines).
</para>
</informalexample>
<para>
The SQL syntax is not very consistent regarding what tokens
......@@ -328,18 +326,16 @@ SELECT 'foo' 'bar';
characters embedded in the constant.
</para>
<informalexample>
<para>
These are some examples of valid floating-point constants:
<literallayout>
<para>
These are some examples of valid floating-point constants:
<literallayout>
3.5
4.
.001
5e2
1.925e-3
</literallayout>
</para>
</informalexample>
</literallayout>
</para>
<para>
Floating-point constants are of type <type>DOUBLE
......@@ -347,10 +343,10 @@ SELECT 'foo' 'bar';
by using <acronym>SQL</acronym> string notation or
<productname>PostgreSQL</productname> type notation:
<programlisting>
<programlisting>
REAL '1.23' -- string style
'1.23'::REAL -- PostgreSQL (historical) style
</programlisting>
</programlisting>
</para>
</sect3>
......@@ -621,6 +617,184 @@ CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
analysis and is effectively replaced by whitespace.
</para>
</sect2>
<sect2 id="sql-precedence">
<title>Lexical Precedence</title>
<indexterm zone="sql-precedence">
<primary>operators</primary>
<secondary>precedence</secondary>
</indexterm>
<para>
The precedence and associativity of the operators is hard-wired
into the parser. Most operators have the same precedence and are
left-associative. This may lead to non-intuitive behavior; for
example the Boolean operators <literal>&lt;</> and <literal>&gt;</> have a different
precedence than the Boolean operators <literal>&lt;=</> and <literal>&gt;=</>. Also,
you will sometimes need to add parentheses when using combinations
of binary and unary operators. For instance
<programlisting>
SELECT 5 ! - 6;
</programlisting>
will be parsed as
<programlisting>
SELECT 5 ! (- 6);
</programlisting>
because the parser has no idea -- until it is too late -- that
<token>!</token> is defined as a postfix operator, not an infix one.
To get the desired behavior in this case, you must write
<programlisting>
SELECT (5 !) - 6;
</programlisting>
This is the price one pays for extensibility.
</para>
<table tocentry="1">
<title>Operator Precedence (decreasing)</title>
<tgroup cols="3">
<thead>
<row>
<entry>Operator/Element</entry>
<entry>Associativity</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><token>.</token></entry>
<entry>left</entry>
<entry>table/column name separator</entry>
</row>
<row>
<entry><token>::</token></entry>
<entry>left</entry>
<entry><productname>PostgreSQL</productname>-style typecast</entry>
</row>
<row>
<entry><token>[</token> <token>]</token></entry>
<entry>left</entry>
<entry>array element selection</entry>
</row>
<row>
<entry><token>-</token></entry>
<entry>right</entry>
<entry>unary minus</entry>
</row>
<row>
<entry><token>^</token></entry>
<entry>left</entry>
<entry>exponentiation</entry>
</row>
<row>
<entry><token>*</token> <token>/</token> <token>%</token></entry>
<entry>left</entry>
<entry>multiplication, division, modulo</entry>
</row>
<row>
<entry><token>+</token> <token>-</token></entry>
<entry>left</entry>
<entry>addition, subtraction</entry>
</row>
<row>
<entry><token>IS</token></entry>
<entry></entry>
<entry>test for TRUE, FALSE, UNKNOWN, NULL</entry>
</row>
<row>
<entry><token>ISNULL</token></entry>
<entry></entry>
<entry>test for NULL</entry>
</row>
<row>
<entry><token>NOTNULL</token></entry>
<entry></entry>
<entry>test for NOT NULL</entry>
</row>
<row>
<entry>(any other)</entry>
<entry>left</entry>
<entry>all other native and user-defined operators</entry>
</row>
<row>
<entry><token>IN</token></entry>
<entry></entry>
<entry>set membership</entry>
</row>
<row>
<entry><token>BETWEEN</token></entry>
<entry></entry>
<entry>containment</entry>
</row>
<row>
<entry><token>OVERLAPS</token></entry>
<entry></entry>
<entry>time interval overlap</entry>
</row>
<row>
<entry><token>LIKE</token> <token>ILIKE</token></entry>
<entry></entry>
<entry>string pattern matching</entry>
</row>
<row>
<entry><token>&lt;</token> <token>&gt;</token></entry>
<entry></entry>
<entry>less than, greater than</entry>
</row>
<row>
<entry><token>=</token></entry>
<entry>right</entry>
<entry>equality, assignment</entry>
</row>
<row>
<entry><token>NOT</token></entry>
<entry>right</entry>
<entry>logical negation</entry>
</row>
<row>
<entry><token>AND</token></entry>
<entry>left</entry>
<entry>logical conjunction</entry>
</row>
<row>
<entry><token>OR</token></entry>
<entry>left</entry>
<entry>logical disjunction</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
Note that the operator precedence rules also apply to user-defined
operators that have the same names as the built-in operators
mentioned above. For example, if you define a
<quote>+</quote> operator for some custom data type it will have
the same precedence as the built-in <quote>+</quote> operator, no
matter what yours does.
</para>
</sect2>
</sect1>
<sect1 id="sql-naming">
......@@ -728,11 +902,11 @@ CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
need to write a qualified operator name in an expression, there is a
special provision: you must write
<synopsis>
<literal>OPERATOR(</><replaceable>schema</><literal>.</><replaceable>operator</><literal>)</>
<literal>OPERATOR(</><replaceable>schema</><literal>.</><replaceable>operator</><literal>)</>
</synopsis>
This is needed to avoid syntactic ambiguity. An example is
<programlisting>
SELECT 3 OPERATOR(pg_catalog.+) 4;
SELECT 3 OPERATOR(pg_catalog.+) 4;
</programlisting>
In practice one usually relies on the search path for operators,
so as not to have to write anything so ugly as that.
......@@ -1256,186 +1430,39 @@ FROM states;
</para>
</sect2>
</sect1>
<sect1 id="sql-precedence">
<title>Lexical Precedence</title>
<sect2>
<title>Expression Evaluation</title>
<indexterm zone="sql-precedence">
<primary>operators</primary>
<secondary>precedence</secondary>
</indexterm>
<para>
The order of evaluation of subexpressions is not defined. In
particular, subexpressions are not necessarily evaluated
left-to-right, right-to-left, or according to the lexical
precedence rules.
</para>
<para>
The precedence and associativity of the operators is hard-wired
into the parser. Most operators have the same precedence and are
left-associative. This may lead to non-intuitive behavior; for
example the Boolean operators <literal>&lt;</> and <literal>&gt;</> have a different
precedence than the Boolean operators <literal>&lt;=</> and <literal>&gt;=</>. Also,
you will sometimes need to add parentheses when using combinations
of binary and unary operators. For instance
Furthermore, if the result of an expression can be determined by
evaluating only some parts of it, then some subexpressions
might not be evaluated at all. For instance, if one wrote
<programlisting>
SELECT 5 ! - 6;
SELECT true OR somefunc();
</programlisting>
will be parsed as
then <literal>somefunc()</literal> would (probably) not be called
at all. The same would be the case if one wrote
<programlisting>
SELECT 5 ! (- 6);
SELECT somefunc() OR true;
</programlisting>
because the parser has no idea -- until it is too late -- that
<token>!</token> is defined as a postfix operator, not an infix one.
To get the desired behavior in this case, you must write
<programlisting>
SELECT (5 !) - 6;
</programlisting>
This is the price one pays for extensibility.
Note that this is not the same as the left-to-right
<quote>short-circuiting</quote> of Boolean operators that is found
in some programming languages.
</para>
<table tocentry="1">
<title>Operator Precedence (decreasing)</title>
<tgroup cols="3">
<thead>
<row>
<entry>Operator/Element</entry>
<entry>Associativity</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><token>.</token></entry>
<entry>left</entry>
<entry>table/column name separator</entry>
</row>
<row>
<entry><token>::</token></entry>
<entry>left</entry>
<entry><productname>PostgreSQL</productname>-style typecast</entry>
</row>
<row>
<entry><token>[</token> <token>]</token></entry>
<entry>left</entry>
<entry>array element selection</entry>
</row>
<row>
<entry><token>-</token></entry>
<entry>right</entry>
<entry>unary minus</entry>
</row>
<row>
<entry><token>^</token></entry>
<entry>left</entry>
<entry>exponentiation</entry>
</row>
<row>
<entry><token>*</token> <token>/</token> <token>%</token></entry>
<entry>left</entry>
<entry>multiplication, division, modulo</entry>
</row>
<row>
<entry><token>+</token> <token>-</token></entry>
<entry>left</entry>
<entry>addition, subtraction</entry>
</row>
<row>
<entry><token>IS</token></entry>
<entry></entry>
<entry>test for TRUE, FALSE, UNKNOWN, NULL</entry>
</row>
<row>
<entry><token>ISNULL</token></entry>
<entry></entry>
<entry>test for NULL</entry>
</row>
<row>
<entry><token>NOTNULL</token></entry>
<entry></entry>
<entry>test for NOT NULL</entry>
</row>
<row>
<entry>(any other)</entry>
<entry>left</entry>
<entry>all other native and user-defined operators</entry>
</row>
<row>
<entry><token>IN</token></entry>
<entry></entry>
<entry>set membership</entry>
</row>
<row>
<entry><token>BETWEEN</token></entry>
<entry></entry>
<entry>containment</entry>
</row>
<row>
<entry><token>OVERLAPS</token></entry>
<entry></entry>
<entry>time interval overlap</entry>
</row>
<row>
<entry><token>LIKE</token> <token>ILIKE</token></entry>
<entry></entry>
<entry>string pattern matching</entry>
</row>
<row>
<entry><token>&lt;</token> <token>&gt;</token></entry>
<entry></entry>
<entry>less than, greater than</entry>
</row>
<row>
<entry><token>=</token></entry>
<entry>right</entry>
<entry>equality, assignment</entry>
</row>
<row>
<entry><token>NOT</token></entry>
<entry>right</entry>
<entry>logical negation</entry>
</row>
<row>
<entry><token>AND</token></entry>
<entry>left</entry>
<entry>logical conjunction</entry>
</row>
<row>
<entry><token>OR</token></entry>
<entry>left</entry>
<entry>logical disjunction</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
Note that the operator precedence rules also apply to user-defined
operators that have the same names as the built-in operators
mentioned above. For example, if you define a
<quote>+</quote> operator for some custom data type it will have
the same precedence as the built-in <quote>+</quote> operator, no
matter what yours does.
As a consequence, it is unwise to use functions with side effects
as part of complex expressions.
</para>
</sect1>
</sect2>
</sect1>
</chapter>
......
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