Support deferrable uniqueness constraints.

The current implementation fires an AFTER ROW trigger for each tuple that
looks like it might be non-unique according to the index contents at the
time of insertion.  This works well as long as there aren't many conflicts,
but won't scale to massive unique-key reassignments.  Improving that case
is a TODO item.

Dean Rasheed

src/backend/commands/constraint.c

@@ -0,0 +1,166 @@
+/*-------------------------------------------------------------------------
+ *
+ * constraint.c
+ *	  PostgreSQL CONSTRAINT support code.
+ *
+ * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  $PostgreSQL: pgsql/src/backend/commands/constraint.c,v 1.1 2009/07/29 20:56:18 tgl Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "catalog/index.h"
+#include "commands/trigger.h"
+#include "executor/executor.h"
+#include "utils/builtins.h"
+#include "utils/tqual.h"
+
+
+/*
+ * unique_key_recheck - trigger function to do a deferred uniqueness check.
+ *
+ * This is invoked as an AFTER ROW trigger for both INSERT and UPDATE,
+ * for any rows recorded as potentially violating a deferrable unique
+ * constraint.
+ *
+ * This may be an end-of-statement check, a commit-time check, or a
+ * check triggered by a SET CONSTRAINTS command.
+ */
+Datum
+unique_key_recheck(PG_FUNCTION_ARGS)
+{
+	TriggerData *trigdata = (TriggerData *) fcinfo->context;
+	const char *funcname = "unique_key_recheck";
+	HeapTuple	new_row;
+	ItemPointerData tmptid;
+	Relation	indexRel;
+	IndexInfo  *indexInfo;
+	EState	   *estate;
+	ExprContext *econtext;
+	TupleTableSlot *slot;
+	Datum		values[INDEX_MAX_KEYS];
+	bool		isnull[INDEX_MAX_KEYS];
+
+	/*
+	 * Make sure this is being called as an AFTER ROW trigger.  Note:
+	 * translatable error strings are shared with ri_triggers.c, so
+	 * resist the temptation to fold the function name into them.
+	 */
+	if (!CALLED_AS_TRIGGER(fcinfo))
+		ereport(ERROR,
+				(errcode(ERRCODE_E_R_I_E_TRIGGER_PROTOCOL_VIOLATED),
+				 errmsg("function \"%s\" was not called by trigger manager",
+						funcname)));
+
+	if (!TRIGGER_FIRED_AFTER(trigdata->tg_event) ||
+		!TRIGGER_FIRED_FOR_ROW(trigdata->tg_event))
+		ereport(ERROR,
+				(errcode(ERRCODE_E_R_I_E_TRIGGER_PROTOCOL_VIOLATED),
+				 errmsg("function \"%s\" must be fired AFTER ROW",
+						funcname)));
+
+	/*
+	 * Get the new data that was inserted/updated.
+	 */
+	if (TRIGGER_FIRED_BY_INSERT(trigdata->tg_event))
+		new_row = trigdata->tg_trigtuple;
+	else if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event))
+		new_row = trigdata->tg_newtuple;
+	else
+	{
+		ereport(ERROR,
+				(errcode(ERRCODE_E_R_I_E_TRIGGER_PROTOCOL_VIOLATED),
+				 errmsg("function \"%s\" must be fired for INSERT or UPDATE",
+						funcname)));
+		new_row = NULL;			/* keep compiler quiet */
+	}
+
+	/*
+	 * If the new_row is now dead (ie, inserted and then deleted within our
+	 * transaction), we can skip the check.  However, we have to be careful,
+	 * because this trigger gets queued only in response to index insertions;
+	 * which means it does not get queued for HOT updates.  The row we are
+	 * called for might now be dead, but have a live HOT child, in which case
+	 * we still need to make the uniqueness check.  Therefore we have to use
+	 * heap_hot_search, not just HeapTupleSatisfiesVisibility as is done in
+	 * the comparable test in RI_FKey_check.
+	 *
+	 * This might look like just an optimization, because the index AM will
+	 * make this identical test before throwing an error.  But it's actually
+	 * needed for correctness, because the index AM will also throw an error
+	 * if it doesn't find the index entry for the row.  If the row's dead then
+	 * it's possible the index entry has also been marked dead, and even
+	 * removed.
+	 */
+	tmptid = new_row->t_self;
+	if (!heap_hot_search(&tmptid, trigdata->tg_relation, SnapshotSelf, NULL))
+	{
+		/*
+		 * All rows in the HOT chain are dead, so skip the check.
+		 */
+		return PointerGetDatum(NULL);
+	}
+
+	/*
+	 * Open the index, acquiring a RowExclusiveLock, just as if we were
+	 * going to update it.  (This protects against possible changes of the
+	 * index schema, not against concurrent updates.)
+	 */
+	indexRel = index_open(trigdata->tg_trigger->tgconstrindid,
+						  RowExclusiveLock);
+	indexInfo = BuildIndexInfo(indexRel);
+
+	/*
+	 * The heap tuple must be put into a slot for FormIndexDatum.
+	 */
+	slot = MakeSingleTupleTableSlot(RelationGetDescr(trigdata->tg_relation));
+
+	ExecStoreTuple(new_row, slot, InvalidBuffer, false);
+
+	/*
+	 * Typically the index won't have expressions, but if it does we need
+	 * an EState to evaluate them.
+	 */
+	if (indexInfo->ii_Expressions != NIL)
+	{
+		estate = CreateExecutorState();
+		econtext = GetPerTupleExprContext(estate);
+		econtext->ecxt_scantuple = slot;
+	}
+	else
+		estate = NULL;
+
+	/*
+	 * Form the index values and isnull flags for the index entry that
+	 * we need to check.
+	 *
+	 * Note: if the index uses functions that are not as immutable as they
+	 * are supposed to be, this could produce an index tuple different from
+	 * the original.  The index AM can catch such errors by verifying that
+	 * it finds a matching index entry with the tuple's TID.
+	 */
+	FormIndexDatum(indexInfo, slot, estate, values, isnull);
+
+	/*
+	 * Now do the uniqueness check. This is not a real insert; it is a
+	 * check that the index entry that has already been inserted is unique.
+	 */
+	index_insert(indexRel, values, isnull, &(new_row->t_self),
+				 trigdata->tg_relation, UNIQUE_CHECK_EXISTING);
+
+	/*
+	 * If that worked, then this index entry is unique, and we are done.
+	 */
+	if (estate != NULL)
+		FreeExecutorState(estate);
+
+	ExecDropSingleTupleTableSlot(slot);
+
+	index_close(indexRel, RowExclusiveLock);
+
+	return PointerGetDatum(NULL);
+}