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Add section explaining unspecified expression evaluation order.

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This commit is contained in:
Peter Eisentraut 2002-06-01 20:56:55 +00:00
parent 1731048c12
commit bf886d5baf
1 changed files with 220 additions and 193 deletions
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413
doc/src/sgml/syntax.sgml
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@ -1,5 +1,5 @@
<!--
$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.
@ -1256,187 +1430,40 @@ FROM states;
</para>
</sect2>
<sect2>
<title>Expression Evaluation</title>
<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>
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 true OR somefunc();
</programlisting>
then <literal>somefunc()</literal> would (probably) not be called
at all. The same would be the case if one wrote
<programlisting>
SELECT somefunc() OR true;
</programlisting>
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>
<para>
As a consequence, it is unwise to use functions with side effects
as part of complex expressions.
</para>
</sect2>
</sect1>
<sect1 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>
</sect1>
</chapter>
<!-- Keep this comment at the end of the file