tableam: Add helper for indexes to check if a corresponding table tuples exist.

This is, likely exclusively, useful to verify that conflicts detected
in a unique index are with live tuples, rather than dead ones.

Author: Andres Freund
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

1 files changed, 34 insertions, 0 deletions

diff --git a/src/backend/access/table/tableam.c b/src/backend/access/table/tableam.c
index b1e31982918..8ad4c629432 100644
--- a/src/backend/access/table/tableam.c
+++ b/src/backend/access/table/tableam.c
@@ -177,6 +177,40 @@ table_beginscan_parallel(Relation relation, ParallelTableScanDesc parallel_scan)
 
 
 /* ----------------------------------------------------------------------------
+ * Index scan related functions.
+ * ----------------------------------------------------------------------------
+ */
+
+/*
+ * To perform that check simply start an index scan, create the necessary
+ * slot, do the heap lookup, and shut everything down again. This could be
+ * optimized, but is unlikely to matter from a performance POV. If there
+ * frequently are live index pointers also matching a unique index key, the
+ * CPU overhead of this routine is unlikely to matter.
+ */
+bool
+table_index_fetch_tuple_check(Relation rel,
+							  ItemPointer tid,
+							  Snapshot snapshot,
+							  bool *all_dead)
+{
+	IndexFetchTableData *scan;
+	TupleTableSlot *slot;
+	bool		call_again = false;
+	bool		found;
+
+	slot = table_slot_create(rel, NULL);
+	scan = table_index_fetch_begin(rel);
+	found = table_index_fetch_tuple(scan, tid, snapshot, slot, &call_again,
+									all_dead);
+	table_index_fetch_end(scan);
+	ExecDropSingleTupleTableSlot(slot);
+
+	return found;
+}
+
+
+/* ----------------------------------------------------------------------------
  * Functions to make modifications a bit simpler.
  * ----------------------------------------------------------------------------
  */