Use PREPARE/EXECUTE for repetitive per-object queries in pg_dump.

For objects such as functions, pg_dump issues the same secondary
data-collection query against each object to be dumped.  This can't
readily be refactored to avoid the repetitive queries, but we can
PREPARE these queries to reduce planning costs.

This patch applies the idea to functions, aggregates, operators, and
data types.  While it could be carried further, the remaining sorts of
objects aren't likely to appear in typical databases enough times to
be worth worrying over.  Moreover, doing the PREPARE is likely to be a
net loss if there aren't at least some dozens of objects to apply the
prepared query to.

Discussion: https://postgr.es/m/7d7eb6128f40401d81b3b7a898b6b4de@W2012-02.nidsa.loc

src/bin/pg_dump/pg_backup.h

@@ -58,6 +58,23 @@ typedef enum _teSection
 	SECTION_POST_DATA			/* stuff to be processed after data */
 } teSection;
 
+/* We need one enum entry per prepared query in pg_dump */
+enum _dumpPreparedQueries
+{
+	PREPQUERY_DUMPAGG,
+	PREPQUERY_DUMPBASETYPE,
+	PREPQUERY_DUMPCOMPOSITETYPE,
+	PREPQUERY_DUMPDOMAIN,
+	PREPQUERY_DUMPENUMTYPE,
+	PREPQUERY_DUMPFUNC,
+	PREPQUERY_DUMPOPR,
+	PREPQUERY_DUMPRANGETYPE,
+	PREPQUERY_DUMPTABLEATTACH,
+	PREPQUERY_GETCOLUMNACLS,
+	PREPQUERY_GETDOMAINCONSTRAINTS,
+	NUM_PREP_QUERIES			/* must be last */
+};
+
 /* Parameters needed by ConnectDatabase; same for dump and restore */
 typedef struct _connParams
 {
@@ -214,6 +231,9 @@ typedef struct Archive
 	bool		exit_on_error;	/* whether to exit on SQL errors... */
 	int			n_errors;		/* number of errors (if no die) */
 
+	/* prepared-query status */
+	bool	   *is_prepared;	/* indexed by enum _dumpPreparedQueries */
+
 	/* The rest is private */
 } Archive;