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Introduce 64-bit hash functions with a 64-bit seed.

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This will be useful for hash partitioning, which needs a way to seed
the hash functions to avoid problems such as a hash index on a hash
partitioned table clumping all values into a small portion of the
bucket space; it's also useful for anything that wants a 64-bit hash
value rather than a 32-bit hash value.

Just in case somebody wants a 64-bit hash value that is compatible
with the existing 32-bit hash values, make the low 32-bits of the
64-bit hash value match the 32-bit hash value when the seed is 0.

Robert Haas and Amul Sul

Discussion: http://postgr.es/m/CA+Tgmoafx2yoJuhCQQOL5CocEi-w_uG4S2xT0EtgiJnPGcHW3g@mail.gmail.com
This commit is contained in:
Robert Haas
2017-08-31 22:21:21 -04:00
parent 2d44c58c79
commit 81c5e46c49
33 changed files with 1555 additions and 42 deletions
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60
src/backend/utils/adt/numeric.c
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@ -2230,6 +2230,66 @@ hash_numeric(PG_FUNCTION_ARGS)
PG_RETURN_DATUM(result);
}
/*
* Returns 64-bit value by hashing a value to a 64-bit value, with a seed.
* Otherwise, similar to hash_numeric.
*/
Datum
hash_numeric_extended(PG_FUNCTION_ARGS)
{
Numeric key = PG_GETARG_NUMERIC(0);
uint64 seed = PG_GETARG_INT64(1);
Datum digit_hash;
Datum result;
int weight;
int start_offset;
int end_offset;
int i;
int hash_len;
NumericDigit *digits;
if (NUMERIC_IS_NAN(key))
PG_RETURN_UINT64(seed);
weight = NUMERIC_WEIGHT(key);
start_offset = 0;
end_offset = 0;
digits = NUMERIC_DIGITS(key);
for (i = 0; i < NUMERIC_NDIGITS(key); i++)
{
if (digits[i] != (NumericDigit) 0)
break;
start_offset++;
weight--;
}
if (NUMERIC_NDIGITS(key) == start_offset)
PG_RETURN_UINT64(seed - 1);
for (i = NUMERIC_NDIGITS(key) - 1; i >= 0; i--)
{
if (digits[i] != (NumericDigit) 0)
break;
end_offset++;
}
Assert(start_offset + end_offset < NUMERIC_NDIGITS(key));
hash_len = NUMERIC_NDIGITS(key) - start_offset - end_offset;
digit_hash = hash_any_extended((unsigned char *) (NUMERIC_DIGITS(key)
+ start_offset),
hash_len * sizeof(NumericDigit),
seed);
result = digit_hash ^ weight;
PG_RETURN_DATUM(result);
}
/* ----------------------------------------------------------------------
*