index 568faf1cd58ee872349dd1c6620a61cacc6197f8..f47eb81012d3ad832a5116330b44288669307241 100644 (file)
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);