Doc: improve description of UNION/INTERSECT/EXCEPT syntax.
queries.sgml failed to mention the rather important point that
INTERSECT binds more tightly than UNION or EXCEPT.  I thought
it could also use more discussion of the role of parentheses
in these constructs.
Per gripe from Christopher Painter-Wakefield.
Discussion:
@@ -1649,17 +1649,9 @@ SELECT DISTINCT ON (<replaceable>expression</replaceable> <optional>, <replaceab
<replaceable>query1</replaceable> INTERSECT <optional>ALL</optional> <replaceable>query2</replaceable>
<replaceable>query1</replaceable> EXCEPT <optional>ALL</optional> <replaceable>query2</replaceable>
</synopsis>
- <replaceable>query1</replaceable> and
+ where <replaceable>query1</replaceable> and
<replaceable>query2</replaceable> are queries that can use any of
- the features discussed up to this point. Set operations can also
- be nested and chained, for example
-<synopsis>
-<replaceable>query1</replaceable> UNION <replaceable>query2</replaceable> UNION <replaceable>query3</replaceable>
-</synopsis>
- which is executed as:
-<synopsis>
-(<replaceable>query1</replaceable> UNION <replaceable>query2</replaceable>) UNION <replaceable>query3</replaceable>
-</synopsis>
+ the features discussed up to this point.
</para>
<para>
@@ -1693,6 +1685,47 @@ SELECT DISTINCT ON (<replaceable>expression</replaceable> <optional>, <replaceab
the corresponding columns have compatible data types, as
described in <xref linkend="typeconv-union-case"/>.
</para>
+
+ <para>
+ Set operations can be combined, for example
+<synopsis>
+<replaceable>query1</replaceable> UNION <replaceable>query2</replaceable> EXCEPT <replaceable>query3</replaceable>
+</synopsis>
+ which is equivalent to
+<synopsis>
+(<replaceable>query1</replaceable> UNION <replaceable>query2</replaceable>) EXCEPT <replaceable>query3</replaceable>
+</synopsis>
+ As shown here, you can use parentheses to control the order of
+ evaluation. Without parentheses, <literal>UNION</literal>
+ and <literal>EXCEPT</literal> associate left-to-right,
+ but <literal>INTERSECT</literal> binds more tightly than those two
+ operators. Thus
+<synopsis>
+<replaceable>query1</replaceable> UNION <replaceable>query2</replaceable> INTERSECT <replaceable>query3</replaceable>
+</synopsis>
+ means
+<synopsis>
+<replaceable>query1</replaceable> UNION (<replaceable>query2</replaceable> INTERSECT <replaceable>query3</replaceable>)
+</synopsis>
+ You can also surround an individual <replaceable>query</replaceable>
+ with parentheses. This is important if
+ the <replaceable>query</replaceable> needs to use any of the clauses
+ discussed in following sections, such as <literal>LIMIT</literal>.
+ Without parentheses, you'll get a syntax error, or else the clause will
+ be understood as applying to the output of the set operation rather
+ than one of its inputs. For example,
+<synopsis>
+SELECT a FROM b UNION SELECT x FROM y LIMIT 10
+</synopsis>
+ is accepted, but it means
+<synopsis>
+(SELECT a FROM b UNION SELECT x FROM y) LIMIT 10
+</synopsis>
+ not
+<synopsis>
+SELECT a FROM b UNION (SELECT x FROM y LIMIT 10)
+</synopsis>
+ </para>
</sect1>
