Fix bug in Tid scan.

Commit 147e3722f7 changed Tid scan so that it calls table_beginscan()
and uses the scan option for seq scan. This change caused two issues.

(1) The change caused Tid scan to take a predicate lock on the entire
       relation in serializable transaction even when relation-level
       lock is not necessary. This could lead to an unexpected
       serialization error.

(2) The change caused Tid scan to increment the number of seq_scan
       in pg_stat_*_tables views even though it's not seq scan. This
       could confuse the users.

This commit adds the scan option for Tid scan and makes Tid scan
use it, to avoid those issues.

Back-patch to v12, where the bug was introduced.

Author: Tatsuhito Kasahara
Reviewed-by: Kyotaro Horiguchi, Masahiko Sawada, Fujii Masao
Discussion: https://postgr.es/m/CAP0=ZVKy+gTbFmB6X_UW0pP3WaeJ-fkUWHoD-pExS=at3CY76g@mail.gmail.com

src/backend/executor/nodeTidscan.c

@@ -143,9 +143,8 @@ TidListEval(TidScanState *tidstate)
 	 */
 	if (tidstate->ss.ss_currentScanDesc == NULL)
 		tidstate->ss.ss_currentScanDesc =
-			table_beginscan(tidstate->ss.ss_currentRelation,
+			table_beginscan_tid(tidstate->ss.ss_currentRelation,
-							tidstate->ss.ps.state->es_snapshot,
+							tidstate->ss.ps.state->es_snapshot);
-							0, NULL);
 	scan = tidstate->ss.ss_currentScanDesc;
 
 	/*

src/backend/utils/adt/tid.c

@@ -381,7 +381,7 @@ currtid_byreloid(PG_FUNCTION_ARGS)
 	ItemPointerCopy(tid, result);
 
 	snapshot = RegisterSnapshot(GetLatestSnapshot());
-	scan = table_beginscan(rel, snapshot, 0, NULL);
+	scan = table_beginscan_tid(rel, snapshot);
 	table_tuple_get_latest_tid(scan, result);
 	table_endscan(scan);
 	UnregisterSnapshot(snapshot);
@@ -419,7 +419,7 @@ currtid_byrelname(PG_FUNCTION_ARGS)
 	ItemPointerCopy(tid, result);
 
 	snapshot = RegisterSnapshot(GetLatestSnapshot());
-	scan = table_beginscan(rel, snapshot, 0, NULL);
+	scan = table_beginscan_tid(rel, snapshot);
 	table_tuple_get_latest_tid(scan, result);
 	table_endscan(scan);
 	UnregisterSnapshot(snapshot);

src/include/access/tableam.h

@@ -47,18 +47,19 @@ typedef enum ScanOptions
 	SO_TYPE_SEQSCAN = 1 << 0,
 	SO_TYPE_BITMAPSCAN = 1 << 1,
 	SO_TYPE_SAMPLESCAN = 1 << 2,
-	SO_TYPE_ANALYZE = 1 << 3,
+	SO_TYPE_TIDSCAN = 1 << 3,
+	SO_TYPE_ANALYZE = 1 << 4,
 
 	/* several of SO_ALLOW_* may be specified */
 	/* allow or disallow use of access strategy */
-	SO_ALLOW_STRAT = 1 << 4,
+	SO_ALLOW_STRAT = 1 << 5,
 	/* report location to syncscan logic? */
-	SO_ALLOW_SYNC = 1 << 5,
+	SO_ALLOW_SYNC = 1 << 6,
 	/* verify visibility page-at-a-time? */
-	SO_ALLOW_PAGEMODE = 1 << 6,
+	SO_ALLOW_PAGEMODE = 1 << 7,
 
 	/* unregister snapshot at scan end? */
-	SO_TEMP_SNAPSHOT = 1 << 7
+	SO_TEMP_SNAPSHOT = 1 << 8
 } ScanOptions;
 
 /*
@@ -829,6 +830,19 @@ table_beginscan_sampling(Relation rel, Snapshot snapshot,
 	return rel->rd_tableam->scan_begin(rel, snapshot, nkeys, key, NULL, flags);
 }
 
+/*
+ * table_beginscan_tid is an alternative entry point for setting up a
+ * TableScanDesc for a Tid scan. As with bitmap scans, it's worth using
+ * the same data structure although the behavior is rather different.
+ */
+static inline TableScanDesc
+table_beginscan_tid(Relation rel, Snapshot snapshot)
+{
+	uint32		flags = SO_TYPE_TIDSCAN;
+
+	return rel->rd_tableam->scan_begin(rel, snapshot, 0, NULL, NULL, flags);
+}
+
 /*
  * table_beginscan_analyze is an alternative entry point for setting up a
  * TableScanDesc for an ANALYZE scan.  As with bitmap scans, it's worth using

src/test/regress/expected/tidscan.out

@@ -277,4 +277,20 @@ SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid;
 (1 row)
 
 RESET enable_hashjoin;
+-- check predicate lock on CTID
+BEGIN ISOLATION LEVEL SERIALIZABLE;
+SELECT * FROM tidscan WHERE ctid = '(0,1)';
+ id 
+----
+  1
+(1 row)
+
+-- locktype should be 'tuple'
+SELECT locktype, mode FROM pg_locks WHERE pid = pg_backend_pid() AND mode = 'SIReadLock';
+ locktype |    mode    
+----------+------------
+ tuple    | SIReadLock
+(1 row)
+
+ROLLBACK;
 DROP TABLE tidscan;

src/test/regress/sql/tidscan.sql

@@ -94,4 +94,11 @@ SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid;
 SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid;
 RESET enable_hashjoin;
 
+-- check predicate lock on CTID
+BEGIN ISOLATION LEVEL SERIALIZABLE;
+SELECT * FROM tidscan WHERE ctid = '(0,1)';
+-- locktype should be 'tuple'
+SELECT locktype, mode FROM pg_locks WHERE pid = pg_backend_pid() AND mode = 'SIReadLock';
+ROLLBACK;
+
 DROP TABLE tidscan;