Replace the hard-wired type knowledge in TypeCategory() and IsPreferredType()

with system catalog lookups, as was foreseen to be necessary almost since
their creation.  Instead put the information into two new pg_type columns,
typcategory and typispreferred.  Add support for setting these when
creating a user-defined base type.

The category column is just a "char" (i.e. a poor man's enum), allowing
a crude form of user extensibility of the category list: just use an
otherwise-unused character.  This seems sufficient for foreseen uses,
but we could upgrade to having an actual category catalog someday, if
there proves to be a huge demand for custom type categories.

In this patch I have attempted to hew exactly to the behavior of the
previous hardwired logic, except for introducing new type categories for
arrays, composites, and enums.  In particular the default preferred state
for user-defined types remains TRUE.  That seems worth revisiting, but it
should be done as a separate patch from introducing the infrastructure.
Likewise, any adjustment of the standard set of categories should be done
separately.

1 files changed, 24 insertions, 1 deletions

diff --git a/src/backend/utils/cache/lsyscache.c b/src/backend/utils/cache/lsyscache.c
index e17bd49e629..79362441f8e 100644
--- a/src/backend/utils/cache/lsyscache.c
+++ b/src/backend/utils/cache/lsyscache.c
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/cache/lsyscache.c,v 1.157 2008/04/13 20:51:21 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/cache/lsyscache.c,v 1.158 2008/07/30 17:05:04 tgl Exp $
  *
  * NOTES
  *	  Eventually, the index information should go through here, too.
@@ -2168,6 +2168,29 @@ type_is_enum(Oid typid)
 }
 
 /*
+ * get_type_category_preferred
+ *
+ *		Given the type OID, fetch its category and preferred-type status.
+ *		Throws error on failure.
+ */
+void
+get_type_category_preferred(Oid typid, char *typcategory, bool *typispreferred)
+{
+	HeapTuple	tp;
+	Form_pg_type typtup;
+
+	tp = SearchSysCache(TYPEOID,
+						ObjectIdGetDatum(typid),
+						0, 0, 0);
+	if (!HeapTupleIsValid(tp))
+		elog(ERROR, "cache lookup failed for type %u", typid);
+	typtup = (Form_pg_type) GETSTRUCT(tp);
+	*typcategory = typtup->typcategory;
+	*typispreferred = typtup->typispreferred;
+	ReleaseSysCache(tp);
+}
+
+/*
  * get_typ_typrelid
  *
  *		Given the type OID, get the typrelid (InvalidOid if not a complex