Add single-item cache when looking at topmost XID of a subtrans XID

This change affects SubTransGetTopmostTransaction(), used to find the
topmost transaction ID of a given transaction ID.  The cache is able to
store one value, so as we can save the backend from unnecessary lookups
at pg_subtrans/ on repetitive calls of this routine.  There is a similar
practice in transam.c, for example.

Author: Simon Riggs
Reviewed-by: Andrey Borodin, Julien Rouhaud
Discussion: https://postgr.es/m/CANbhV-G8Co=yq4v4BkW7MJDqVt68K_8A48nAZ_+8UQS7LrwLEQ@mail.gmail.com

src/backend/access/transam/subtrans.c

@@ -54,6 +54,14 @@
 #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;
 }