From ade493e02d25f4807c02dcd763a25d4232b3b68d Mon Sep 17 00:00:00 2001
From: Neil Conway <neilc@samurai.com>
Date: Tue, 8 May 2007 18:56:48 +0000
Subject: Add a hash function for "numeric". Mark the equality operator for
 numerics as "oprcanhash", and make the corresponding system catalog updates.
 As a result, hash indexes, hashed aggregation, and hash joins can now be used
 with the numeric type. Bump the catversion.

The only tricky aspect to doing this is writing a correct hash
function: it's possible for two Numerics to be equal according to
their equality operator, but have different in-memory bit patterns.
To cope with this, the hash function doesn't consider the Numeric's
"scale" or "sign", and explictly skips any leading or trailing
zeros in the Numeric's digit buffer (the current implementation
should suppress any such zeros, but it seems unwise to rely upon
this). See discussion on pgsql-patches for more details.
---
 src/backend/utils/adt/numeric.c | 78 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 77 insertions(+), 1 deletion(-)

(limited to 'src/backend/utils/adt/numeric.c')

diff --git a/src/backend/utils/adt/numeric.c b/src/backend/utils/adt/numeric.c
index f173f30211c..6a6f3d44afb 100644
--- a/src/backend/utils/adt/numeric.c
+++ b/src/backend/utils/adt/numeric.c
@@ -14,7 +14,7 @@
  * Copyright (c) 1998-2007, PostgreSQL Global Development Group
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/adt/numeric.c,v 1.101 2007/02/27 23:48:08 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/adt/numeric.c,v 1.102 2007/05/08 18:56:47 neilc Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -26,6 +26,7 @@
 #include <limits.h>
 #include <math.h>
 
+#include "access/hash.h"
 #include "catalog/pg_type.h"
 #include "libpq/pqformat.h"
 #include "utils/array.h"
@@ -1149,6 +1150,81 @@ cmp_numerics(Numeric num1, Numeric num2)
 	return result;
 }
 
+Datum
+hash_numeric(PG_FUNCTION_ARGS)
+{
+	Numeric 	key = PG_GETARG_NUMERIC(0);
+	Datum 		digit_hash;
+	Datum 		result;
+	int 		weight;
+	int 		start_offset;
+	int 		end_offset;
+	int 		i;
+	int 		hash_len;
+
+	/* If it's NaN, don't try to hash the rest of the fields */
+	if (NUMERIC_IS_NAN(key))
+		PG_RETURN_UINT32(0);
+
+	weight 		 = key->n_weight;
+	start_offset = 0;
+	end_offset 	 = 0;
+
+	/*
+	 * Omit any leading or trailing zeros from the input to the
+	 * hash. The numeric implementation *should* guarantee that
+	 * leading and trailing zeros are suppressed, but we're
+	 * paranoid. Note that we measure the starting and ending offsets
+	 * in units of NumericDigits, not bytes.
+	 */
+	for (i = 0; i < NUMERIC_NDIGITS(key); i++)
+	{
+		if (NUMERIC_DIGITS(key)[i] != (NumericDigit) 0)
+			break;
+
+		start_offset++;
+		/*
+		 * The weight is effectively the # of digits before the
+		 * decimal point, so decrement it for each leading zero we
+		 * skip.
+		 */
+		weight--;
+	}
+
+	/*
+	 * If there are no non-zero digits, then the value of the number
+	 * is zero, regardless of any other fields.
+	 */
+	if (NUMERIC_NDIGITS(key) == start_offset)
+		PG_RETURN_UINT32(-1);
+
+	for (i = NUMERIC_NDIGITS(key) - 1; i >= 0; i--)
+	{
+		if (NUMERIC_DIGITS(key)[i] != (NumericDigit) 0)
+			break;
+
+		end_offset++;
+	}
+
+	/* If we get here, there should be at least one non-zero digit */
+	Assert(start_offset + end_offset < NUMERIC_NDIGITS(key));
+
+	/*
+	 * Note that we don't hash on the Numeric's scale, since two
+	 * numerics can compare equal but have different scales. We also
+	 * don't hash on the sign, although we could: since a sign
+	 * difference implies inequality, this shouldn't affect correctness.
+	 */
+	hash_len = NUMERIC_NDIGITS(key) - start_offset - end_offset;
+	digit_hash = hash_any((unsigned char *) (NUMERIC_DIGITS(key) + start_offset),
+                          hash_len * sizeof(NumericDigit));
+
+	/* Mix in the weight, via XOR */
+	result = digit_hash ^ weight;
+
+	PG_RETURN_DATUM(result);
+}
+
 
 /* ----------------------------------------------------------------------
  *
-- 
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