Fix performance problems in multi-batch hash joins by ensuring that we select

a well-randomized batch number even when given a poorly-randomized hash value.
This is a bit inefficient but seems the only practical solution given the
constraint that we can't change the hash functions in released branches.
Per report from Joseph Shraibman.

Applied to 8.1 and 8.2 only --- HEAD is getting a cleaner fix, and 8.0 and
before use different coding that seems less vulnerable.

diff --git a/src/backend/access/hash/hashfunc.c b/src/backend/access/hash/hashfunc.c
index a4803708ad439eecba2f7157c87f965dd5cb1cfc..f03df3d599953498f23e0f81e9bd30fe3f1a1146 100644 (file)
--- a/src/backend/access/hash/hashfunc.c
+++ b/src/backend/access/hash/hashfunc.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *       $PostgreSQL: pgsql/src/backend/access/hash/hashfunc.c,v 1.45.2.1 2005/12/22 22:50:06 tgl Exp $
+ *       $PostgreSQL: pgsql/src/backend/access/hash/hashfunc.c,v 1.45.2.2 2007/06/01 15:58:09 tgl Exp $
  *
  * NOTES
  *       These functions are stored in pg_amproc.      For each operator class
@@ -267,6 +267,31 @@ hash_any(register const unsigned char *k, register int keylen)
                        /* case 0: nothing left to add */
        }
        mix(a, b, c);
+
+       /* report the result */
+       return UInt32GetDatum(c);
+}
+
+/*
+ * hash_uint32() -- hash a 32-bit value
+ *
+ * This has the same result (at least on little-endian machines) as
+ *             hash_any(&k, sizeof(uint32))
+ * but is faster and doesn't force the caller to store k into memory.
+ */
+Datum
+hash_uint32(uint32 k)
+{
+       register uint32 a,
+                               b,
+                               c;
+
+       a = 0x9e3779b9 + k;
+       b = 0x9e3779b9;
+       c = 3923095 + (uint32) sizeof(uint32);
+
+       mix(a, b, c);
+
        /* report the result */
        return UInt32GetDatum(c);
 }

diff --git a/src/backend/executor/nodeHash.c b/src/backend/executor/nodeHash.c
index a40423179d14d808ca9677b9af2e7af2a7aeec9f..826cafe87373df29bc3978d2641ee56621f10491 100644 (file)
--- a/src/backend/executor/nodeHash.c
+++ b/src/backend/executor/nodeHash.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *       $PostgreSQL: pgsql/src/backend/executor/nodeHash.c,v 1.96.2.2 2005/11/23 20:28:04 tgl Exp $
+ *       $PostgreSQL: pgsql/src/backend/executor/nodeHash.c,v 1.96.2.3 2007/06/01 15:58:09 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -20,6 +20,7 @@
  */
 #include "postgres.h"
 
+#include "access/hash.h"
 #include "executor/execdebug.h"
 #include "executor/hashjoin.h"
 #include "executor/instrument.h"
@@ -713,9 +714,11 @@ ExecHashGetHashValue(HashJoinTable hashtable,
  * chains), and must only cause the batch number to remain the same or
  * increase.  Our algorithm is
  *             bucketno = hashvalue MOD nbuckets
- *             batchno = (hashvalue DIV nbuckets) MOD nbatch
- * where nbuckets should preferably be prime so that all bits of the
- * hash value can affect both bucketno and batchno.
+ *             batchno = hash_uint32(hashvalue) MOD nbatch
+ * which gives reasonably independent bucket and batch numbers in the face
+ * of some rather poorly-implemented hash functions in hashfunc.c.  (This
+ * will change in PG 8.3.)
+ *
  * nbuckets doesn't change over the course of the join.
  *
  * nbatch is always a power of 2; we increase it only by doubling it.  This
@@ -734,7 +737,7 @@ ExecHashGetBucketAndBatch(HashJoinTable hashtable,
        {
                *bucketno = hashvalue % nbuckets;
                /* since nbatch is a power of 2, can do MOD by masking */
-               *batchno = (hashvalue / nbuckets) & (nbatch - 1);
+               *batchno = hash_uint32(hashvalue) & (nbatch - 1);
        }
        else
        {

diff --git a/src/include/access/hash.h b/src/include/access/hash.h
index 03d2fc6c12e36d26faf6788905323a1a7959e403..27861bb7243a3e36e6c6ce7243db2c53802b542d 100644 (file)
--- a/src/include/access/hash.h
+++ b/src/include/access/hash.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/hash.h,v 1.63.2.1 2007/04/19 20:24:18 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/hash.h,v 1.63.2.2 2007/06/01 15:58:09 tgl Exp $
  *
  * NOTES
  *             modeled after Margo Seltzer's hash implementation for unix.
@@ -263,6 +263,7 @@ extern Datum hashname(PG_FUNCTION_ARGS);
 extern Datum hashtext(PG_FUNCTION_ARGS);
 extern Datum hashvarlena(PG_FUNCTION_ARGS);
 extern Datum hash_any(register const unsigned char *k, register int keylen);
+extern Datum hash_uint32(uint32 k);
 
 /* private routines */