Function initHyperLogLogError

src/backend/lib/hyperloglog.c:127–142

* Initialize HyperLogLog track state, by error rate * * Instead of specifying bwidth (number of bits used for addressing the * register), this method allows sizing the counter for particular error * rate using a simple formula from the paper: * * e = 1.04 / sqrt(m) * * where 'm' is the number of registers, i.e. (2^bwidth). The method * finds the lowest bwidth with 'e' below the requested

 * is between ~25% (bwidth=4) and 0.4% (bwidth=16).
 */
void
initHyperLogLogError(hyperLogLogState *cState, double error)
{
	uint8		bwidth = 4;

	while (bwidth < 16)
	{
		double		m = (Size) 1 << bwidth;

		if (1.04 / sqrt(m) < error)
			break;
		bwidth++;
	}

	initHyperLogLog(cState, bwidth);
}

/*
 * Free HyperLogLog track state