Change currtid functions to use an MVCC snapshot, not SnapshotNow.

This has a slight performance cost, but the only known consumers
of these functions, known at the SQL level as currtid and currtid2,
is pgsql-odbc; whose usage, we hope, is not sufficiently intensive
to make this a problem.

Per discussion.

diff --git a/src/backend/utils/adt/tid.c b/src/backend/utils/adt/tid.c
index 5df9e399a9702831b6d0b2c65dc97cc0335a706d..1c2503f50f6ecc0413496368a703b6b597ab54ad 100644 (file)
--- a/src/backend/utils/adt/tid.c
+++ b/src/backend/utils/adt/tid.c
@@ -30,6 +30,7 @@
 #include "utils/acl.h"
 #include "utils/builtins.h"
 #include "utils/rel.h"
+#include "utils/snapmgr.h"
 #include "utils/tqual.h"
 
 
@@ -332,6 +333,7 @@ currtid_byreloid(PG_FUNCTION_ARGS)
    ItemPointer result;
    Relation    rel;
    AclResult   aclresult;
+   Snapshot    snapshot;
 
    result = (ItemPointer) palloc(sizeof(ItemPointerData));
    if (!reloid)
@@ -352,7 +354,10 @@ currtid_byreloid(PG_FUNCTION_ARGS)
        return currtid_for_view(rel, tid);
 
    ItemPointerCopy(tid, result);
-   heap_get_latest_tid(rel, SnapshotNow, result);
+
+   snapshot = RegisterSnapshot(GetLatestSnapshot());
+   heap_get_latest_tid(rel, snapshot, result);
+   UnregisterSnapshot(snapshot);
 
    heap_close(rel, AccessShareLock);
 
@@ -368,6 +373,7 @@ currtid_byrelname(PG_FUNCTION_ARGS)
    RangeVar   *relrv;
    Relation    rel;
    AclResult   aclresult;
+   Snapshot    snapshot;
 
    relrv = makeRangeVarFromNameList(textToQualifiedNameList(relname));
    rel = heap_openrv(relrv, AccessShareLock);
@@ -384,7 +390,9 @@ currtid_byrelname(PG_FUNCTION_ARGS)
    result = (ItemPointer) palloc(sizeof(ItemPointerData));
    ItemPointerCopy(tid, result);
 
-   heap_get_latest_tid(rel, SnapshotNow, result);
+   snapshot = RegisterSnapshot(GetLatestSnapshot());
+   heap_get_latest_tid(rel, snapshot, result);
+   UnregisterSnapshot(snapshot);
 
    heap_close(rel, AccessShareLock);