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Introduce bloom_filter_size for BRIN bloom opclass 

Move the calculation of Bloom filter parameters (for BRIN indexes) into
a separate function to make reuse easier. At the moment we only call it
from one place, but that may change and it's easier to read anyway.

Reviewed-by: Heikki Linnakangas
Discussion: https://postgr.es/m/0e1f3350-c9cf-ab62-43a5-5dae314de89c%40enterprisedb.com
This commit is contained in:
Tomas Vondra 2023-07-02 10:24:13 +02:00
parent 28d03feac3
commit 2b8b2852bb
1 changed files with 47 additions and 16 deletions
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63
src/backend/access/brin/brin_bloom.c
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@ -259,6 +259,48 @@ typedef struct BloomFilter
char data[FLEXIBLE_ARRAY_MEMBER];
} BloomFilter;
/*
* bloom_filter_size
* Calculate Bloom filter parameters (nbits, nbytes, nhashes).
*
* Given expected number of distinct values and desired false positive rate,
* calculates the optimal parameters of the Bloom filter.
*
* The resulting parameters are returned through nbytesp (number of bytes),
* nbitsp (number of bits) and nhashesp (number of hash functions). If a
* pointer is NULL, the parameter is not returned.
*/
static void
bloom_filter_size(int ndistinct, double false_positive_rate,
int *nbytesp, int *nbitsp, int *nhashesp)
{
double k;
int nbits,
nbytes;
/* sizing bloom filter: -(n * ln(p)) / (ln(2))^2 */
nbits = ceil(-(ndistinct * log(false_positive_rate)) / pow(log(2.0), 2));
/* round m to whole bytes */
nbytes = ((nbits + 7) / 8);
nbits = nbytes * 8;
/*
* round(log(2.0) * m / ndistinct), but assume round() may not be
* available on Windows
*/
k = log(2.0) * nbits / ndistinct;
k = (k - floor(k) >= 0.5) ? ceil(k) : floor(k);
if (nbytesp)
*nbytesp = nbytes;
if (nbitsp)
*nbitsp = nbits;
if (nhashesp)
*nhashesp = (int) k;
}
/*
* bloom_init
@ -275,19 +317,15 @@ bloom_init(int ndistinct, double false_positive_rate)
int nbits; /* size of filter / number of bits */
int nbytes; /* size of filter / number of bytes */
double k; /* number of hash functions */
int nhashes; /* number of hash functions */
Assert(ndistinct > 0);
Assert((false_positive_rate >= BLOOM_MIN_FALSE_POSITIVE_RATE) &&
(false_positive_rate < BLOOM_MAX_FALSE_POSITIVE_RATE));
/* sizing bloom filter: -(n * ln(p)) / (ln(2))^2 */
nbits = ceil(-(ndistinct * log(false_positive_rate)) / pow(log(2.0), 2));
/* round m to whole bytes */
nbytes = ((nbits + 7) / 8);
nbits = nbytes * 8;
/* calculate bloom filter size / parameters */
bloom_filter_size(ndistinct, false_positive_rate,
&nbytes, &nbits, &nhashes);
/*
* Reject filters that are obviously too large to store on a page.
@ -310,13 +348,6 @@ bloom_init(int ndistinct, double false_positive_rate)
elog(ERROR, "the bloom filter is too large (%d > %zu)", nbytes,
BloomMaxFilterSize);
/*
* round(log(2.0) * m / ndistinct), but assume round() may not be
* available on Windows
*/
k = log(2.0) * nbits / ndistinct;
k = (k - floor(k) >= 0.5) ? ceil(k) : floor(k);
/*
* We allocate the whole filter. Most of it is going to be 0 bits, so the
* varlena is easy to compress.
@ -326,7 +357,7 @@ bloom_init(int ndistinct, double false_positive_rate)
filter = (BloomFilter *) palloc0(len);
filter->flags = 0;
filter->nhashes = (int) k;
filter->nhashes = nhashes;
filter->nbits = nbits;
SET_VARSIZE(filter, len);