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

This reverts commit 06f5295 as per issues with this approach, both in
terms of efficiency impact and stability.  First, contrary to the
single-item cache for transaction IDs in transam.c, the cache may finish
by not be hit for a long time, and without an invalidation mechanism to
clear it, it would cause inconsistent results on wraparound for
example.  Second, the use of SubTransGetTopmostTransaction() for the
caching has a limited impact on performance.  SubTransGetParent() could
have more impact, though the benchmarking of the single-item approach
still needs to be proved, particularly under the conditions where SLRU
lookups are stressed in parallel with overflowed snapshots (aka more
than 64 subxids generated, for example).

After discussion with Andres Freund.

Discussion: https://postgr.es/m/20220524235250[email protected]

diff --git a/src/backend/access/transam/subtrans.c b/src/backend/access/transam/subtrans.c
index 7240454ca4e5b33d1d2acfa487a713512c9e4160..66d3548155248b2cdb9cba16a01344885d1103fb 100644 (file)
--- a/src/backend/access/transam/subtrans.c
+++ b/src/backend/access/transam/subtrans.c
@@ -54,14 +54,6 @@
 #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
@@ -163,13 +155,6 @@ 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;
@@ -189,9 +174,6 @@ SubTransGetTopmostTransaction(TransactionId xid)
 
    Assert(TransactionIdIsValid(previousXid));
 
-   cachedFetchSubXid = xid;
-   cachedFetchTopmostXid = previousXid;
-
    return previousXid;
 }