index 66d3548155248b2cdb9cba16a01344885d1103fb..7240454ca4e5b33d1d2acfa487a713512c9e4160 100644 (file)
#define TransactionIdToPage(xid) ((xid) / (TransactionId) SUBTRANS_XACTS_PER_PAGE)
#define TransactionIdToEntry(xid) ((xid) % (TransactionId) SUBTRANS_XACTS_PER_PAGE)
+/*
+ * Single-item cache for results of SubTransGetTopmostTransaction(). It's
+ * worth having such a cache because we frequently find ourselves repeatedly
+ * checking the same XID, for example when scanning a table just after a
+ * bulk insert, update, or delete.
+ */
+static TransactionId cachedFetchSubXid = InvalidTransactionId;
+static TransactionId cachedFetchTopmostXid = InvalidTransactionId;
/*
* Link to shared-memory data structures for SUBTRANS control
@@ -155,6 +163,13 @@ SubTransGetTopmostTransaction(TransactionId xid)
/* Can't ask about stuff that might not be around anymore */
Assert(TransactionIdFollowsOrEquals(xid, TransactionXmin));
+ /*
+ * Before going to the subtrans log, check our single item cache to see if
+ * we know the result from a previous/recent request.
+ */
+ if (TransactionIdEquals(xid, cachedFetchSubXid))
+ return cachedFetchTopmostXid;
+
while (TransactionIdIsValid(parentXid))
{
previousXid = parentXid;
@@ -174,6 +189,9 @@ SubTransGetTopmostTransaction(TransactionId xid)
Assert(TransactionIdIsValid(previousXid));
+ cachedFetchSubXid = xid;
+ cachedFetchTopmostXid = previousXid;
+
return previousXid;
}