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 1e2d779a14cfe58d914a126326d6714bbb1758f3..1f9813277a388206794091070c1f511505204a0d 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 1.48 2006/10/04 00:29:48 momjian Exp $
+ *       $PostgreSQL: pgsql/src/backend/access/hash/hashfunc.c,v 1.48.2.1 2007/06/01 15:58:01 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 171cfb58f287286470998a0d77674d3c313f2aaf..b36832b8384a451d396b317e2a3e214efa7b2fd3 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 1.107 2006/07/14 04:44:46 momjian Exp $
+ *       $PostgreSQL: pgsql/src/backend/executor/nodeHash.c,v 1.107.2.1 2007/06/01 15:58:01 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -24,6 +24,7 @@
 #include <math.h>
 #include <limits.h>
 
+#include "access/hash.h"
 #include "executor/execdebug.h"
 #include "executor/hashjoin.h"
 #include "executor/instrument.h"
@@ -719,9 +720,11 @@ 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
@@ -740,7 +743,7 @@ 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 016e51d09f9b6dd5d87f1aaa909f06b52d299031..4b8ce7889280e923210dde758ff281258b69e1ad 100644 (file)
--- a/src/include/access/hash.h
+++ b/src/include/access/hash.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/hash.h,v 1.73.2.1 2007/04/19 20:24:10 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/hash.h,v 1.73.2.2 2007/06/01 15:58:02 tgl Exp $
  *
  * NOTES
  *             modeled after Margo Seltzer's hash implementation for unix.
@@ -262,6 +262,7 @@ 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 */