Thread: Adding LIMIT changes PostgreSQL plan from good to a bad one

Hi everybody.

I stumbled upon a weird problem with the query planner. I have a query
on a typical EAV schema:

SELECT contacts.id
FROM contacts
    LEFT OUTER JOIN (SELECT DISTINCT contacts_values.company_id AS
company_id, contacts_values.id AS id
                     FROM contacts_values
                     WHERE contacts_values.field_id =
'\x000000000000000000000000'
                       AND contacts_values.field_name = 'facebook'
                       AND
nimble_contact_value_normalize(nimble_skip_long_values(contacts_values.value))
=

nimble_contact_value_normalize('http://www.facebook.com/jon.ferrara'))
AS anon_1
                    ON anon_1.company_id = contacts.company_id AND
anon_1.id = contacts.id
    LEFT OUTER JOIN (SELECT DISTINCT contacts_values.company_id AS
company_id, contacts_values.id AS id
                     FROM contacts_values
                     WHERE contacts_values.field_id =
'\x000000000000000000000000'
                       AND contacts_values.field_name = 'last_name'
                       AND
nimble_contact_value_normalize(nimble_skip_long_values(contacts_values.value))
=
                           nimble_contact_value_normalize('Ferrara')) AS anon_2
                    ON anon_2.company_id = contacts.company_id AND
anon_2.id = contacts.id
    JOIN contacts__aggregated AS contacts__aggregated_1
         ON contacts__aggregated_1.company_id = contacts.company_id AND
            contacts__aggregated_1.contact_id = contacts.id AND
contacts__aggregated_1.field_name = 'names'
WHERE contacts.company_id = '\x4c2118ad54397f271b000000'
  AND (anon_1.id IS NOT NULL OR anon_2.id IS NOT NULL)
ORDER BY contacts__aggregated_1.value ASC
LIMIT 30 OFFSET 0;

My problem is that the LIMIT clause in this query makes the planner
choose a bad plan
with nested loops: https://explain.depesz.com/s/Mute. Running the same
query after
SET ENABLE_NESTLOOP TO OFF I am getting a much more efficient plan:
https://explain.depesz.com/s/b5kn. Removing the LIMIT from the query
results in a
similar plan: https://explain.depesz.com/s/wDqE.

One thing that concerns me is that the cost of the LIMIT node in the
bad nested loop
plan is just a fraction of the cost of its subnode. But for the better
merge join
plan LIMIT node has the same cost as its subnode. How could it be this
way? And what
can I do to make the planner pick up a better plan?

We are running PostgreSQL 10.10.

Sincerely,

-- Michael Korbakov

On November 21, 2019 at 19:14:33, Pavel Stehule (pavel.stehule@gmail.com) wrote:

čt 21. 11. 2019 v 17:19 odesílatel Michael Korbakov <rmihael@gmail.com> napsal:

Hi everybody.

I stumbled upon a weird problem with the query planner. I have a query
on a typical EAV schema:

SELECT contacts.id
FROM contacts
    LEFT OUTER JOIN (SELECT DISTINCT contacts_values.company_id AS
company_id, contacts_values.id AS id
                     FROM contacts_values
                     WHERE contacts_values.field_id =
'\x000000000000000000000000'
                       AND contacts_values.field_name = 'facebook'
                       AND
nimble_contact_value_normalize(nimble_skip_long_values(contacts_values.value))
=

nimble_contact_value_normalize('http://www.facebook.com/jon.ferrara'))
AS anon_1
                    ON anon_1.company_id = contacts.company_id AND
anon_1.id = contacts.id
    LEFT OUTER JOIN (SELECT DISTINCT contacts_values.company_id AS
company_id, contacts_values.id AS id
                     FROM contacts_values
                     WHERE contacts_values.field_id =
'\x000000000000000000000000'
                       AND contacts_values.field_name = 'last_name'
                       AND
nimble_contact_value_normalize(nimble_skip_long_values(contacts_values.value))
=
                           nimble_contact_value_normalize('Ferrara')) AS anon_2
                    ON anon_2.company_id = contacts.company_id AND
anon_2.id = contacts.id
    JOIN contacts__aggregated AS contacts__aggregated_1
         ON contacts__aggregated_1.company_id = contacts.company_id AND
            contacts__aggregated_1.contact_id = contacts.id AND
contacts__aggregated_1.field_name = 'names'
WHERE contacts.company_id = '\x4c2118ad54397f271b000000'
  AND (anon_1.id IS NOT NULL OR anon_2.id IS NOT NULL)
ORDER BY contacts__aggregated_1.value ASC
LIMIT 30 OFFSET 0;

My problem is that the LIMIT clause in this query makes the planner
choose a bad plan
with nested loops: https://explain.depesz.com/s/Mute. Running the same
query after
SET ENABLE_NESTLOOP TO OFF I am getting a much more efficient plan:
https://explain.depesz.com/s/b5kn. Removing the LIMIT from the query
results in a
similar plan: https://explain.depesz.com/s/wDqE.

One thing that concerns me is that the cost of the LIMIT node in the
bad nested loop
plan is just a fraction of the cost of its subnode. But for the better
merge join
plan LIMIT node has the same cost as its subnode. How could it be this
way? And what
can I do to make the planner pick up a better plan?

We are running PostgreSQL 10.10.

Sometimes are problems with LIMIT clause, because it too much decrease costs. The system expects so necessary values are found quickly - but if this premise is not valid, then this plan can be bad.

LIMIT node with nested loop decreases the cost proportionally to requested number of rows versus estimated number. That may be too aggressive, but the same LIMIT node with merge join does not decrease total cost at all. I do not understand why is it happening. 

typical solution is wrapping to subquery and using OFFSET 0 (that is optimizer fence)

SELECT * FROM foo WHERE x = 10 LIMIT 10 -- should be transformed

SELECT * FROM (SELECT * FROM foo WHERE x = 10 OFFSET 0) s LIMIT 10;

Unfortunately, that has not worked for me (https://explain.depesz.com/s/slsM). Looks like OFFSET 0 is not working as a fence. However, I  managed to get some success with wrapping everything except LIMIT into a CTE: https://explain.depesz.com/s/n7c4. 

But you can see in explain very bad estimations - left join is estimated to 918K rows and result is just 83 rows

There is relative very high rows removed in top part of query "Rows Removed by Filter: 1043891"

It's EAV table - it is unoptimized pattern :/

Is there any way to tune planner to choose better plan for such queries? I tried increasing default_statistics_target to 1000 and creating extended statistics: CREATE STATISTICS contacts_values_company_id_field (dependencies) ON company_id, field_id, field_name FROM contacts_values. After running ANALYZE on all relevant tables I noticed no changes in planner's behavior.

Pavel

Sincerely,

-- Michael Korbakov