Re: Query plan for views and WHERE clauses, Luke is not using the index

[kimaidou <kimaidou@xxxxxxxxx>]

Hi all

I also tried to change the values of join_collapse_limit and rom_collapse_limit to higher values than default: 12, 50 or even 100, with no improvement on the query plan.

Is this a typical behavior, or is there something particular in my query that causes this big difference between the raw query and the view with WHERE ?

Regards

Michaël

2017-08-18 18:46 GMT+02:00 kimaidou <kimaidou@xxxxxxxxx>:

Hi all,

I have come across a unexpected behavior.

You can see full detail on an issue on the QGEP project in Github :

https://github.com/QGEP/QGEP/issues/308#issuecomment-323122514

Basically, we have this view with some LEFT JOIN :
http://paste.debian.net/982003/

We have indexes on some fields ( foreign keys, and a GIST index for the PostGIS geometry field)

If I use the raw SQL defining the view, and add a WHERE clause like:

WHERE "progression_geometry" && st_makeenvelope(1728327.03249295568093657,8240789.26074041239917278,1728608.10987572139129043,8240958.16933418624103069,3949)

the query plan is "as expected", as it is using the spatial index (and others too). This query gets 100 lines from a "main" table containing 20000 lines (and child tables having more). It is pretty fast and "low cost"

See the query plan:

https://explain.depesz.com/s/6Qgb

When we call the WHERE on the view:

EXPLAIN (ANALYZE, COSTS, VERBOSE, BUFFERS)
SELECT *
FROM "qgep"."vw_qgep_reach"
WHERE "progression_geometry" && st_makeenvelope(1728327.03249295568093657,8240789.26074041239917278,1728608.10987572139129043,8240958.16933418624103069,3949)

The query plan is "wrong", as PostgreSQL seems to consider it should do a seq scan on the tables, and only afterwards filter with the WHERE:

https://explain.depesz.com/s/wXV

The query takes about 1 second instead of less than 100ms.

Do you have any hint on this kind of issue ?

Thanks in advance

Regards,

Michaël