Add SQL-accessible functions for inspecting index AM properties.

Per discussion, we should provide such functions to replace the lost
ability to discover AM properties by inspecting pg_am (cf commit
65c5fcd35).  The added functionality is also meant to displace any code
that was looking directly at pg_index.indoption, since we'd rather not
believe that the bit meanings in that field are part of any client API
contract.

As future-proofing, define the SQL API to not assume that properties that
are currently AM-wide or index-wide will remain so unless they logically
must be; instead, expose them only when inquiring about a specific index
or even specific index column.  Also provide the ability for an index
AM to override the behavior.

In passing, document pg_am.amtype, overlooked in commit 473b93287.

Andrew Gierth, with kibitzing by me and others

Discussion: <87mvl5on7n.fsf@news-spur.riddles.org.uk>

1 files changed, 24 insertions, 3 deletions

diff --git a/src/backend/access/index/amapi.c b/src/backend/access/index/amapi.c
index d347ebcba4..28f6cde896 100644
--- a/src/backend/access/index/amapi.c
+++ b/src/backend/access/index/amapi.c
@@ -47,9 +47,12 @@ GetIndexAmRoutine(Oid amhandler)
 /*
  * GetIndexAmRoutineByAmId - look up the handler of the index access method
  * with the given OID, and get its IndexAmRoutine struct.
+ *
+ * If the given OID isn't a valid index access method, returns NULL if
+ * noerror is true, else throws error.
  */
 IndexAmRoutine *
-GetIndexAmRoutineByAmId(Oid amoid)
+GetIndexAmRoutineByAmId(Oid amoid, bool noerror)
 {
 	HeapTuple	tuple;
 	Form_pg_am	amform;
@@ -58,25 +61,43 @@ GetIndexAmRoutineByAmId(Oid amoid)
 	/* Get handler function OID for the access method */
 	tuple = SearchSysCache1(AMOID, ObjectIdGetDatum(amoid));
 	if (!HeapTupleIsValid(tuple))
+	{
+		if (noerror)
+			return NULL;
 		elog(ERROR, "cache lookup failed for access method %u",
 			 amoid);
+	}
 	amform = (Form_pg_am) GETSTRUCT(tuple);
 
-	/* Check if it's index access method */
+	/* Check if it's an index access method as opposed to some other AM */
 	if (amform->amtype != AMTYPE_INDEX)
+	{
+		if (noerror)
+		{
+			ReleaseSysCache(tuple);
+			return NULL;
+		}
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 				 errmsg("access method \"%s\" is not of type %s",
 						NameStr(amform->amname), "INDEX")));
+	}
 
 	amhandler = amform->amhandler;
 
 	/* Complain if handler OID is invalid */
 	if (!RegProcedureIsValid(amhandler))
+	{
+		if (noerror)
+		{
+			ReleaseSysCache(tuple);
+			return NULL;
+		}
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 				 errmsg("index access method \"%s\" does not have a handler",
 						NameStr(amform->amname))));
+	}
 
 	ReleaseSysCache(tuple);
 
@@ -107,7 +128,7 @@ amvalidate(PG_FUNCTION_ARGS)
 
 	ReleaseSysCache(classtup);
 
-	amroutine = GetIndexAmRoutineByAmId(amoid);
+	amroutine = GetIndexAmRoutineByAmId(amoid, false);
 
 	if (amroutine->amvalidate == NULL)
 		elog(ERROR, "function amvalidate is not defined for index access method %u",