Fix vac_update_relstats to ensure it always sends a relcache inval message,

even if none of the fields in the pg_class row change.  This behavior is
necessary to ensure other backends flush rd_targblock values that might
point to truncated-away pages.  We got this right pre-8.2 but it was broken
by overoptimistic change to not write out the pg_class row if unchanged.
Per report from Pavan Deolasee.

diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index 58c171f22306e92f5b94978a1a8aeffd61673e71..3729c662a4dad496f092f2b660286bde7977f474 100644 (file)
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -715,10 +715,20 @@ vac_update_relstats(Oid relid, BlockNumber num_pages, double num_tuples,
        }
 
        /*
-        * If anything changed, write out the tuple
+        * If anything changed, write out the tuple.  Even if nothing changed,
+        * force relcache invalidation so all backends reset their rd_targblock
+        * --- otherwise it might point to a page we truncated away.
         */
        if (dirty)
+       {
                heap_inplace_update(rd, ctup);
+               /* the above sends a cache inval message */
+       }
+       else
+       {
+               /* no need to change tuple, but force relcache inval anyway */
+               CacheInvalidateRelcacheByTuple(ctup);
+       }
 
        heap_close(rd, RowExclusiveLock);
 }