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From: Sbob <[email protected]>
To: [email protected] <[email protected]>
Subject: checking for a NULL date in a partitioned table kills performance
Date: Thu, 22 Aug 2024 15:44:49 -0600
Message-ID: <[email protected]> (raw)

All;

I am running a select from a partitioned table. The table (and all the 
partitions) have an index on contract_date like this:
CREATE INDEX on part_tab (contract_date) where contract_date > 
'2022-01-01'::date

The table (including all partitions) has 32million rows
The db server is an aurora postgresql instance with 128GB of ram and 16 
vcpu's

The shared buffers is set to 90GB and effective_cache_size is also 90GB
I set default_statistics_target to 1000 and ram a vacuum analyze on the 
table

I am selecting a number of columns and specifying this where clause:

WHERE (
                     (contract_date IS NULL)
                     OR
                     (contract_date > '2022-01-01'::date)
              )

This takes 15 seconds to run and an explain says it's doing a table scan 
on all partitions (the query is not specifying the partition key)
If I change the where clause to look like this:

WHERE (
                   (contract_date > '2022-01-01'::date)
              )

Then it performs index scans on all the partitions and runs in about 600ms

If i leave the where clause off entirely it performs table scans of the 
partitions and takes approx 18 seconds to run

I am trying to get the performance to less than 2sec,
I have tried adding indexes on the table and all partitions like this:
CREATE INDEX ON table (contract_date NULLS FIRST) ;
but the performance with the full where clause is the same:

WHERE (
                     (contract_date IS NULL)
                     OR
                     (contract_date > '2022-01-01'::date)
              )

runs in 15 seconds and scans all partitions

I also tried indexes i=on the table and all partitions like this:
CREATE INDEX ON table (contract_date) WHERE contract_date IS NULL;

but I get the same result, table scans on all partitions and it runs in 
15 seconds

Any help or advice ?

Thanks in advance

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