On Sat, Aug 07, 2021 at 07:35:25PM +, 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
> 0.0002334 | 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
