Here are my tables:

 - id

 - account_id: FK to company
 - viewer_id: FK to company

 - companyview_id: FK to companyview
 - is_shipper: boolean (not null)

 - original_id: FK to companyview
 - similar_id: FK to companyview 

This is my query (generated by Django)

SELECT COUNT(*) AS "__count"
  FROM "accounts_similarcompanyview"
 INNER JOIN "accounts_companyview"
    ON ("accounts_similarcompanyview"."original_id" = "accounts_companyview"."id")
 INNER JOIN "shipments_address"
    ON ("accounts_companyview"."id" = "shipments_address"."company_view_id")
 INNER JOIN "accounts_companyview" T5
    ON ("accounts_similarcompanyview"."similar_id" = T5."id")
 INNER JOIN "shipments_address" T6
    ON (T5."id" = T6."company_view_id")
 WHERE ("accounts_similarcompanyview"."are_similar" = true AND "shipments_address"."is_shipper" = true AND "accounts_companyview"."viewer_id" = 51729 AND T6."is_shipper" = true AND T5."viewer_id" = 51729);

Output of EXPLAIN ANALYZE: https://explain.depesz.com/s/slJK

I can see that there is an Index Scan taking most of the time, so I tried to add a few indexes but no change whatsoever, I still get a full index scan.

CREATE INDEX "shipments_a_is_ship_d78ee4_idx" ON "shipments_address" ("is_shipper");
CREATE INDEX "shipments_a_is_ship_d78ee4_idx" ON "shipments_address" ("is_shipper", "company_view_id");

What index could I add to make this query faster?

EDIT: forgot to add the size of the tables

select count(*) from shipments_address;

select count(*) from accounts_company;

select count(*) from accounts_companyview;

select count(*) from accounts_similarcompanyview;
  • Could you please enable IO statistics: SET STATISTICS IO ON; and post the message from SSMS after running your query with it enabled? And since you're working with views it's possible that something on the view code (that was made for a different purpose ohter than the result of this specific query) is causing the scan. – Ronaldo Oct 21 '19 at 17:03
  • 1
    @Ronaldo: You are talking MS SQL Server, but this is about Postgres. – Erwin Brandstetter Oct 21 '19 at 17:07
  • Sorry for my lack of attention, you're right. But I believe my point about the view having code that was meant to attend another query being the reason of your scans still stands. I don't believe Postgres differs from SQL Server on that point. Was the view created for this specific query? – Ronaldo Oct 21 '19 at 17:16
  • In PostgreSQL, that is spelled track_io_timing. – jjanes Oct 21 '19 at 17:55
  • @Ronaldo I don't think I'm working with views, accounts_companyview is the name of the table. Or maybe I misunderstood something? – Corentin S. Oct 22 '19 at 7:13

The main problem are the bad estimates of the join cardinalities that lead PostgreSQL to use a nested loop join when a hash join would perform better.

There is one simple thing you can do to reduce the impact of the outermost nested loop join:

CREATE INDEX ON shipments_address (company_view_id) WHERE is_shipper;

That should cut the execution time roughly in half.

Other than that, I can think of the ugly method of temporarily disabling nested loop joins:


SET LOCAL enable_nestloop = off;

/* your query */

| improve this answer | |

You are not getting a full index scan. You are getting a regular (partial or parameterized) index scan, repeated 52,190 times. Each individual scan is very fast (I'm not sure how much it can be improved) but when you do it that many times it adds up.

It might be faster to do a hash join against that table rather than the nested loop index scan. It depends on how big the table is. You can drop the index "shipments_address_company_view_id_1119f23e" and see what happens. Or you could turn up random_page_cost (just in this one session) and see if that forces the switch, and whether it is better once it does.

CREATE INDEX "shipments_a_is_ship_d78ee4_idx" ON "shipments_address" ("is_shipper", "company_view_id");

I would expect that that index would be used. Although I wouldn't expect using it to improve your performance dramatically--at most 50%, certainly not 10 fold. Also, I would suggest swapping the order of the columns. Either way would work for this particular query, but the putting the boolean last will probably make it usable for a greater variety of queries and allow you to drop the index on (company_view_id) alone.

Perhaps you can drop the other index and see if this one gets used then. If you don't have a test/QA database setup and don't want to really drop the index in production, you can drop it inside a transaction, do the EXPLAIN and then roll it back. Other users will be locked out of your table from the DROP until the ROLLBACK, but if you put all the commands into a file, or put them all on the command line on one line, and if do a plain EXPLAIN rather than an EXPLAIN ANALYZE, this should be a small fraction of a second.

| improve this answer | |
  • Thanks, I ran it on my staging DB with the index reversed: "company_view_id", "is_shipper" and it took it from 75s to 16s: explain.depesz.com/s/yDXq – Corentin S. Oct 22 '19 at 7:11

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