Reference material for HLL_COUNT_ESTIMATE
HLL_COUNT_ESTIMATE(<expression>)
<expression>
BYTEA
BIGINT
SELECT hll_count_estimate(a) AS hll_estimate FROM sketch_of_data_to_count ORDER BY 1;
SELECT hll_count_estimate(hll_count_merge(a)) AS hll_estimate FROM sketch_of_data_to_count;