Query Discrepancy in Postgres HLL Test
Hi PostgreSQL Community,
I'm currently delving into Postgres HLL (HyperLogLog) functionality and
have encountered an unexpected behavior while executing queries from the "
cumulative_add_sparse_edge.sql
<https://github.com/citusdata/postgresql-hll/blob/master/sql/cumulative_add_sparse_edge.sql#L28-L36>"
regress test. This particular test data file
<https://github.com/citusdata/postgresql-hll/blob/master/sql/data/cumulative_add_sparse_edge.csv#L515-L516>
involves
three columns, with the last column representing an HLL (HyperLogLog) value
derived from the previous HLL value and the current raw value.
Upon manual inspection of the query responsible for deriving the last row's
HLL value, I noticed a discrepancy. When executing the query:
"""
-- '\x148B481002....' is second last rows hll value
SELECT hll_add('\x148B481002.....', hll_hashval(2561));
"""
instead of obtaining the expected value (''\x148B481002....''), I received
a different output which is ('\x138b48000200410061008100a1 ........').
I am using
postgres=> select version();
version
-------------------------------------------------------------------------------------------------------------
PostgreSQL 16.1 on aarch64-unknown-linux-gnu, compiled by
aarch64-unknown-linux-gnu-gcc (GCC) 9.5.0, 64-bit
My initial assumption is that this could potentially be attributed to a
precision error. However, I'm reaching out to seek clarity on why this
disparity is occurring and to explore potential strategies for mitigating
it (as I want the behaviour to be consistent to regress test file).
Regards
Ayush Vatsa
On Wed, May 1, 2024 at 1:10 PM Ayush Vatsa <ayushvatsa1810@gmail.com> wrote:
I'm currently delving into Postgres HLL (HyperLogLog) functionality and have encountered an unexpected behavior while executing queries from the "cumulative_add_sparse_edge.sql" regress test. This particular test data file involves three columns, with the last column representing an HLL (HyperLogLog) value derived from the previous HLL value and the current raw value.
Upon manual inspection of the query responsible for deriving the last row's HLL value, I noticed a discrepancy. When executing the query:
"""
-- '\x148B481002....' is second last rows hll value
SELECT hll_add('\x148B481002.....', hll_hashval(2561));
"""
instead of obtaining the expected value (''\x148B481002....''), I received a different output which is ('\x138b48000200410061008100a1 ........').
PostgreSQL has no function called hll_add or hll_hashval, and no
regression test file called cumulative_add_sparse_edge.sql. A quick
Google search suggests that these things are part of citusdata's fork
of PostgreSQL, so you might want to contact them.
--
Robert Haas
EDB: http://www.enterprisedb.com