Document that "Q" is ignored by to_date and to_timestamp. Add C comment

about the behavior.

Document that quotes in to_date, to_timestamp, to_number skip input
characters.

doc/src/sgml/func.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.506 2010/02/23 16:14:25 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.507 2010/03/03 22:28:42 momjian Exp $ -->
 
  <chapter id="functions">
   <title>Functions and Operators</title>
@@ -5089,7 +5089,7 @@ SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
        </row>
        <row>
         <entry><literal>Q</literal></entry>
-        <entry>quarter</entry>
+        <entry>quarter (ignored by <function>to_date</> and <function>to_timestamp</>)</entry>
        </row>
        <row>
         <entry><literal>RM</literal></entry>
@@ -5209,7 +5209,10 @@ SELECT SUBSTRING('XY1234Z', 'Y*?([0-9]{1,3})');
        even if it contains pattern key words.  For example, in
        <literal>'"Hello Year "YYYY'</literal>, the <literal>YYYY</literal>
        will be replaced by the year data, but the single <literal>Y</literal> in <literal>Year</literal>
-       will not be.
+       will not be.  In <function>to_date</>, <function>to_number</>,
+       and <function>to_timestamp</>, double-quoted strings skip the number of
+       input characters contained in the string, e.g. <literal>"XX"</>
+       skips two input characters.
       </para>
      </listitem>
 

src/backend/utils/adt/formatting.c

 /* -----------------------------------------------------------------------
  * formatting.c
  *
- * $PostgreSQL: pgsql/src/backend/utils/adt/formatting.c,v 1.168 2010/02/26 02:01:08 momjian Exp $
+ * $PostgreSQL: pgsql/src/backend/utils/adt/formatting.c,v 1.169 2010/03/03 22:28:42 momjian Exp $
  *
  *
  *	 Portions Copyright (c) 1999-2010, PostgreSQL Global Development Group
@@ -2671,10 +2671,12 @@ DCH_from_char(FormatNode *node, char *in, TmFromChar *out)
 				s += SKIP_THth(n->suffix);
 				break;
 			case DCH_Q:
-
 				/*
-				 * We ignore Q when converting to date because it is not
-				 * normative.
+				 * We ignore 'Q' when converting to date because it is
+				 * unclear which date in the quarter to use, and some
+				 * people specify both quarter and month, so if it was
+				 * honored it might conflict with the supplied month.
+				 * That is also why we don't throw an error.
 				 *
 				 * We still parse the source string for an integer, but it
 				 * isn't stored anywhere in 'out'.