Further twiddling of nodeHash.c hashtable sizing calculation.

On reflection, the submitted patch didn't really work to prevent the
request size from exceeding MaxAllocSize, because of the fact that we'd
happily round nbuckets up to the next power of 2 after we'd limited it to
max_pointers.  The simplest way to enforce the limit correctly is to
round max_pointers down to a power of 2 when it isn't one already.

(Note that the constraint to INT_MAX / 2, if it were doing anything useful
at all, is properly applied after that.)

diff --git a/src/backend/executor/nodeHash.c b/src/backend/executor/nodeHash.c
index 492e6e0d3324d390af34bb597dfe5dda6d87a4fa..560505f56424a1a88036e2850fe745718521d8c1 100644 (file)
--- a/src/backend/executor/nodeHash.c
+++ b/src/backend/executor/nodeHash.c
@@ -396,6 +396,7 @@ ExecChooseHashTableSize(double ntuples, int tupwidth, bool useskew,
        long            hash_table_bytes;
        long            skew_table_bytes;
        long            max_pointers;
+       long            mppow2;
        int                     nbatch;
        int                     nbuckets;
        int                     i;
@@ -463,7 +464,12 @@ ExecChooseHashTableSize(double ntuples, int tupwidth, bool useskew,
         */
        max_pointers = (work_mem * 1024L) / sizeof(HashJoinTuple);
        max_pointers = Min(max_pointers, MaxAllocSize / sizeof(HashJoinTuple));
-       /* also ensure we avoid integer overflow in nbatch and nbuckets */
+       /* If max_pointers isn't a power of 2, must round it down to one */
+       mppow2 = 1L << my_log2(max_pointers);
+       if (max_pointers != mppow2)
+               max_pointers = mppow2 / 2;
+
+       /* Also ensure we avoid integer overflow in nbatch and nbuckets */
        /* (this step is redundant given the current value of MaxAllocSize) */
        max_pointers = Min(max_pointers, INT_MAX / 2);