Introduce logical decoding.

This feature, building on previous commits, allows the write-ahead log
stream to be decoded into a series of logical changes; that is,
inserts, updates, and deletes and the transactions which contain them.
It is capable of handling decoding even across changes to the schema
of the effected tables.  The output format is controlled by a
so-called "output plugin"; an example is included.  To make use of
this in a real replication system, the output plugin will need to be
modified to produce output in the format appropriate to that system,
and to perform filtering.

Currently, information can be extracted from the logical decoding
system only via SQL; future commits will add the ability to stream
changes via walsender.

Andres Freund, with review and other contributions from many other
people, including Álvaro Herrera, Abhijit Menon-Sen, Peter Gheogegan,
Kevin Grittner, Robert Haas, Heikki Linnakangas, Fujii Masao, Abhijit
Menon-Sen, Michael Paquier, Simon Riggs, Craig Ringer, and Steve
Singer.

5 files changed, 26 insertions, 9 deletions

diff --git a/src/backend/commands/analyze.c b/src/backend/commands/analyze.c
index e7fcb558684..a04adeaac75 100644
--- a/src/backend/commands/analyze.c
+++ b/src/backend/commands/analyze.c
@@ -22,6 +22,7 @@
 #include "access/tuptoaster.h"
 #include "access/visibilitymap.h"
 #include "access/xact.h"
+#include "catalog/catalog.h"
 #include "catalog/index.h"
 #include "catalog/indexing.h"
 #include "catalog/pg_collation.h"
@@ -1081,7 +1082,7 @@ acquire_sample_rows(Relation onerel, int elevel,
 	totalblocks = RelationGetNumberOfBlocks(onerel);
 
 	/* Need a cutoff xmin for HeapTupleSatisfiesVacuum */
-	OldestXmin = GetOldestXmin(onerel->rd_rel->relisshared, true);
+	OldestXmin = GetOldestXmin(onerel, true);
 
 	/* Prepare for sampling block numbers */
 	BlockSampler_Init(&bs, totalblocks, targrows);
diff --git a/src/backend/commands/cluster.c b/src/backend/commands/cluster.c
index 8b18e4acb72..b6b40e724e7 100644
--- a/src/backend/commands/cluster.c
+++ b/src/backend/commands/cluster.c
@@ -850,7 +850,7 @@ copy_heap_data(Oid OIDNewHeap, Oid OIDOldHeap, Oid OIDOldIndex, bool verbose,
 	 * Since we're going to rewrite the whole table anyway, there's no reason
 	 * not to be aggressive about this.
 	 */
-	vacuum_set_xid_limits(0, 0, 0, 0, OldHeap->rd_rel->relisshared,
+	vacuum_set_xid_limits(OldHeap, 0, 0, 0, 0,
 						  &OldestXmin, &FreezeXid, NULL, &MultiXactCutoff,
 						  NULL);
 
@@ -869,7 +869,7 @@ copy_heap_data(Oid OIDNewHeap, Oid OIDOldHeap, Oid OIDOldIndex, bool verbose,
 	is_system_catalog = IsSystemRelation(OldHeap);
 
 	/* Initialize the rewrite operation */
-	rwstate = begin_heap_rewrite(NewHeap, OldestXmin, FreezeXid,
+	rwstate = begin_heap_rewrite(OldHeap, NewHeap, OldestXmin, FreezeXid,
 								 MultiXactCutoff, use_wal);
 
 	/*
diff --git a/src/backend/commands/dbcommands.c b/src/backend/commands/dbcommands.c
index 5d540aa3a01..4996a2e7cd2 100644
--- a/src/backend/commands/dbcommands.c
+++ b/src/backend/commands/dbcommands.c
@@ -45,6 +45,7 @@
 #include "miscadmin.h"
 #include "pgstat.h"
 #include "postmaster/bgwriter.h"
+#include "replication/slot.h"
 #include "storage/copydir.h"
 #include "storage/fd.h"
 #include "storage/lmgr.h"
@@ -750,6 +751,7 @@ dropdb(const char *dbname, bool missing_ok)
 	HeapTuple	tup;
 	int			notherbackends;
 	int			npreparedxacts;
+	int			nslots, nslots_active;
 
 	/*
 	 * Look up the target database's OID, and get exclusive lock on it. We
@@ -807,6 +809,19 @@ dropdb(const char *dbname, bool missing_ok)
 				 errmsg("cannot drop the currently open database")));
 
 	/*
+	 * Check whether there are, possibly unconnected, logical slots that refer
+	 * to the to-be-dropped database. The database lock we are holding
+	 * prevents the creation of new slots using the database.
+	 */
+	if (ReplicationSlotsCountDBSlots(db_id, &nslots, &nslots_active))
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_IN_USE),
+				 errmsg("database \"%s\" is used by a logical decoding slot",
+						dbname),
+				 errdetail("There are %d slot(s), %d of them active",
+						   nslots, nslots_active)));
+
+	/*
 	 * Check for other backends in the target database.  (Because we hold the
 	 * database lock, no new ones can start after this.)
 	 *
diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index 5ae7763534b..ded1841dc65 100644
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -398,11 +398,11 @@ get_rel_oids(Oid relid, const RangeVar *vacrel)
  * not interested.
  */
 void
-vacuum_set_xid_limits(int freeze_min_age,
+vacuum_set_xid_limits(Relation rel,
+					  int freeze_min_age,
 					  int freeze_table_age,
 					  int multixact_freeze_min_age,
 					  int multixact_freeze_table_age,
-					  bool sharedRel,
 					  TransactionId *oldestXmin,
 					  TransactionId *freezeLimit,
 					  TransactionId *xidFullScanLimit,
@@ -425,7 +425,7 @@ vacuum_set_xid_limits(int freeze_min_age,
 	 * working on a particular table at any time, and that each vacuum is
 	 * always an independent transaction.
 	 */
-	*oldestXmin = GetOldestXmin(sharedRel, true);
+	*oldestXmin = GetOldestXmin(rel, true);
 
 	Assert(TransactionIdIsNormal(*oldestXmin));
 
@@ -795,7 +795,7 @@ vac_update_datfrozenxid(void)
 	 * committed pg_class entries for new tables; see AddNewRelationTuple().
 	 * So we cannot produce a wrong minimum by starting with this.
 	 */
-	newFrozenXid = GetOldestXmin(true, true);
+	newFrozenXid = GetOldestXmin(NULL, true);
 
 	/*
 	 * Similarly, initialize the MultiXact "min" with the value that would be
diff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.c
index d77892ee7f8..d5db917d97f 100644
--- a/src/backend/commands/vacuumlazy.c
+++ b/src/backend/commands/vacuumlazy.c
@@ -44,6 +44,7 @@
 #include "access/multixact.h"
 #include "access/transam.h"
 #include "access/visibilitymap.h"
+#include "catalog/catalog.h"
 #include "catalog/storage.h"
 #include "commands/dbcommands.h"
 #include "commands/vacuum.h"
@@ -204,10 +205,10 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
 
 	vac_strategy = bstrategy;
 
-	vacuum_set_xid_limits(vacstmt->freeze_min_age, vacstmt->freeze_table_age,
+	vacuum_set_xid_limits(onerel,
+						  vacstmt->freeze_min_age, vacstmt->freeze_table_age,
 						  vacstmt->multixact_freeze_min_age,
 						  vacstmt->multixact_freeze_table_age,
-						  onerel->rd_rel->relisshared,
 						  &OldestXmin, &FreezeLimit, &xidFullScanLimit,
 						  &MultiXactCutoff, &mxactFullScanLimit);