tableam: introduce table AM infrastructure.

This introduces the concept of table access methods, i.e. CREATE
  ACCESS METHOD ... TYPE TABLE and
  CREATE TABLE ... USING (storage-engine).
No table access functionality is delegated to table AMs as of this
commit, that'll be done in following commits.

Subsequent commits will incrementally abstract table access
functionality to be routed through table access methods. That change
is too large to be reviewed & committed at once, so it'll be done
incrementally.

Docs will be updated at the end, as adding them incrementally would
likely make them less coherent, and definitely is a lot more work,
without a lot of benefit.

Table access methods are specified similar to index access methods,
i.e. pg_am.amhandler returns, as INTERNAL, a pointer to a struct with
callbacks. In contrast to index AMs that struct needs to live as long
as a backend, typically that's achieved by just returning a pointer to
a constant struct.

Psql's \d+ now displays a table's access method. That can be disabled
with HIDE_TABLEAM=true, which is mainly useful so regression tests can
be run against different AMs.  It's quite possible that this behaviour
still needs to be fine tuned.

For now it's not allowed to set a table AM for a partitioned table, as
we've not resolved how partitions would inherit that. Disallowing
allows us to introduce, if we decide that's the way forward, such a
behaviour without a compatibility break.

Catversion bumped, to add the heap table AM and references to it.

Author: Haribabu Kommi, Andres Freund, Alvaro Herrera, Dimitri Golgov and others
Discussion:
    https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de
    https://postgr.es/m/20160812231527.GA690404@alvherre.pgsql
    https://postgr.es/m/20190107235616.6lur25ph22u5u5av@alap3.anarazel.de
    https://postgr.es/m/20190304234700.w5tmhducs5wxgzls@alap3.anarazel.de

2 files changed, 17 insertions, 1 deletions

diff --git a/src/include/utils/rel.h b/src/include/utils/rel.h
index 1d05465303..9d805ca23d 100644
--- a/src/include/utils/rel.h
+++ b/src/include/utils/rel.h
@@ -124,6 +124,20 @@ typedef struct RelationData
 	 */
 	bytea	   *rd_options;		/* parsed pg_class.reloptions */
 
+	/*
+	 * Oid of the handler for this relation. For an index this is a function
+	 * returning IndexAmRoutine, for table like relations a function returning
+	 * TableAmRoutine.  This is stored separately from rd_indam, rd_tableam as
+	 * its lookup requires syscache access, but during relcache bootstrap we
+	 * need to be able to initialize rd_tableam without syscache lookups.
+	 */
+	Oid			rd_amhandler;	/* OID of index AM's handler function */
+
+	/*
+	 * Table access method.
+	 */
+	const struct TableAmRoutine *rd_tableam;
+
 	/* These are non-NULL only for an index relation: */
 	Form_pg_index rd_index;		/* pg_index tuple describing this index */
 	/* use "struct" here to avoid needing to include htup.h: */
@@ -144,7 +158,6 @@ typedef struct RelationData
 	 * rd_indexcxt.  A relcache reset will include freeing that chunk and
 	 * setting rd_amcache = NULL.
 	 */
-	Oid			rd_amhandler;	/* OID of index AM's handler function */
 	MemoryContext rd_indexcxt;	/* private memory cxt for this stuff */
 	/* use "struct" here to avoid needing to include amapi.h: */
 	struct IndexAmRoutine *rd_indam;	/* index AM's API struct */
diff --git a/src/include/utils/relcache.h b/src/include/utils/relcache.h
index a80e335374..8f5bd67649 100644
--- a/src/include/utils/relcache.h
+++ b/src/include/utils/relcache.h
@@ -75,6 +75,8 @@ extern void RelationInitIndexAccessInfo(Relation relation);
 struct PublicationActions;
 extern struct PublicationActions *GetRelationPublicationActions(Relation relation);
 
+extern void RelationInitTableAccessMethod(Relation relation);
+
 /*
  * Routines to support ereport() reports of relation-related errors
  */
@@ -97,6 +99,7 @@ extern Relation RelationBuildLocalRelation(const char *relname,
 						   Oid relnamespace,
 						   TupleDesc tupDesc,
 						   Oid relid,
+						   Oid accessmtd,
 						   Oid relfilenode,
 						   Oid reltablespace,
 						   bool shared_relation,