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

1 files changed, 40 insertions, 0 deletions

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index a93b13c2fe4..788544ec928 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -21,6 +21,7 @@
 #include "access/reloptions.h"
 #include "access/relscan.h"
 #include "access/sysattr.h"
+#include "access/tableam.h"
 #include "access/tupconvert.h"
 #include "access/xact.h"
 #include "access/xlog.h"
@@ -537,6 +538,8 @@ DefineRelation(CreateStmt *stmt, char relkind, Oid ownerId,
 	Oid			ofTypeId;
 	ObjectAddress address;
 	LOCKMODE	parentLockmode;
+	const char *accessMethod = NULL;
+	Oid			accessMethodId = InvalidOid;
 
 	/*
 	 * Truncate relname to appropriate length (probably a waste of time, as
@@ -778,6 +781,42 @@ DefineRelation(CreateStmt *stmt, char relkind, Oid ownerId,
 	}
 
 	/*
+	 * If the statement hasn't specified an access method, but we're defining
+	 * a type of relation that needs one, use the default.
+	 */
+	if (stmt->accessMethod != NULL)
+	{
+		accessMethod = stmt->accessMethod;
+
+		if (relkind == RELKIND_PARTITIONED_TABLE)
+			ereport(ERROR,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("specifying a table access method is not supported on a partitioned table")));
+
+	}
+	else if (relkind == RELKIND_RELATION ||
+			 relkind == RELKIND_TOASTVALUE ||
+			 relkind == RELKIND_MATVIEW)
+		accessMethod = default_table_access_method;
+
+	/*
+	 * look up the access method, verify it can handle the requested features
+	 */
+	if (accessMethod != NULL)
+	{
+		HeapTuple	tuple;
+
+		tuple = SearchSysCache1(AMNAME, PointerGetDatum(accessMethod));
+		if (!HeapTupleIsValid(tuple))
+				ereport(ERROR,
+						(errcode(ERRCODE_UNDEFINED_OBJECT),
+						 errmsg("table access method \"%s\" does not exist",
+								 accessMethod)));
+		accessMethodId = ((Form_pg_am) GETSTRUCT(tuple))->oid;
+		ReleaseSysCache(tuple);
+	}
+
+	/*
 	 * Create the relation.  Inherited defaults and constraints are passed in
 	 * for immediate handling --- since they don't need parsing, they can be
 	 * stored immediately.
@@ -789,6 +828,7 @@ DefineRelation(CreateStmt *stmt, char relkind, Oid ownerId,
 										  InvalidOid,
 										  ofTypeId,
 										  ownerId,
+										  accessMethodId,
 										  descriptor,
 										  list_concat(cookedDefaults,
 													  old_constraints),