Re: Adding non-selective key to jsonb query @> reduces performance?

Marcin Krupowicz <ma(at)rcin(dot)me> writes:
> However this one, is slow:

> Q2
> select count(*) from tbl where row @> '{"SELECTIVE_COL":
> "SearchValue", "DATE": "20220606", "NON_SELECTIVE_COL": "Abc"}'::jsonb
> It takes 17ms

> Note that the only difference is adding one more - not very unique -
> key. If in Q2 I replaced NON_SELECTIVE_COL with another selective
> column, it's becoming fast again.

This doesn't surprise me a whole lot based on what I know of GIN.
It's going to store sets of TIDs associated with each key or value
mentioned in the data, and then a query will have to AND the sets
of TIDs for keys/values mentioned in the query. That will take
longer when some of those sets are big.

It might be worth experimenting with an index built using the
non-default jsonb_path_ops opclass [1]. I'm not sure if that'd be
faster for this scenario, but it seems worth trying.

regards, tom lane

[1] https://www.postgresql.org/docs/current/datatype-json.html#JSON-INDEXING