Date: Sat, 7 Aug 2021 19:35:28 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: Alex <cdalxndr@yahoo.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Slow query because lexeme index not used
Message-ID: <20210808003528.GZ10479@telsasoft.com>
References: <1557921338.669347.1628364925994.ref@mail.yahoo.com>
 <1557921338.669347.1628364925994@mail.yahoo.com>
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On Sat, Aug 07, 2021 at 07:35:25PM +0000, Alex wrote:
> Table "product" has a GIN index on "lexeme" column (tsvector) that is not used.
> 
> Query that doesn't use lexeme idx:  https://explain.dalibo.com/plan/BlB#plan, ~8s, ~60.000 blocks needed
> 
> Query forced to use lexeme idx: https://explain.dalibo.com/plan/i52, ~800ms (10x less), ~15.000 blocks needed (x4 less)

Could you show the table stats for product.id ?  In particular its
"correlation".

I guess the correlation is ~1, and the 10,659 index scans on product.id are
considered to be cheaper than scannning the lexeme index - since there are no
correlation stats for tsvector.

How large is shared_buffers ?

Does the query plan improve if you increase work_mem ?

Maybe you could encourage scanning in order of product_property.product.
You could CLUSTER product_property_default on an index on "product" and then
ANALYZE.  Or you could write the query with a temp table:

CREATE TEMP TABLE product_ids AS
SELECT product
FROM product_property
WHERE "meaning" = 'B' AND "first" = 1.7179869184E10
GROUP BY 1 -- or DISTINCT, because the table is only used for EXISTS
ORDER BY 1; -- to scan product in order of id
ANALYZE product_ids;

The index scans on product.id should be faster when you use
EXISTS(SELECT 1 FROM product_ids ...), even though it didn't use the lexeme index.

Maybe it would help to create stats on "first" and "meaning"; the rowcount is
underestimated by 3x, which means it did several times more index scans into
"product" than planned.
| Bitmap Heap Scan on product_property_default product_property_default (cost=2,748.6..8,823.4 rows=6,318 width=4) (actual time=43.945..211.621 rows=21,061 loops=1) 

CREATE STATISTICS first_meaning ON first,meaning FROM product_property;
ANALYZE product_property;

> Table metdata:
>          relname          | relpages | reltuples | relallvisible | relkind | relnatts | relhassubclass | reloptions | pg_table_size
> --------------------------+----------+-----------+---------------+---------+----------+----------------+------------+---------------
>  product_property_default |     8992 |    622969 |          8992 | r       |       16 | f              |            |      73719808
>  product                  |    49686 |    413840 |         49686 | r       |       14 | f              |            |     493314048
>
> Table stats:
>    frac_mcv    |        tablename         | attname | inherited | null_frac | n_distinct  | n_mcv | n_hist | correlation
> ---------------+--------------------------+---------+-----------+-----------+-------------+-------+--------+-------------
>                | product                  | lexeme  | f         |         0 |          -1 |       |        |
>     0.99773335 | product_property_default | meaning | f         |         0 |          63 |    39 |     24 |  0.19444875
>      0.6416333 | product_property_default | first   | f         |         0 |        2193 |   100 |    101 | -0.09763639
>  0.00023333334 | product_property_default | product | f         |         0 | -0.15221785 |     1 |    101 |  0.08643274
> 
> 
> Using windows docker with wsl2.Both cases are run with cold cache.All database memory is limited to 1GB by using .wslconfig file with memory=1GB, also the docker container is limited to 1GB. 
> My requirement is to optimize disk access with this limited memory