slow order by "field" and limit - postgresql

Question

I have simple query that must get 1 record from table with about 14m records:
EXPLAIN ANALYZE SELECT "projects_toolresult"."id",
"projects_toolresult"."tool_id",
"projects_toolresult"."status",
"projects_toolresult"."updated_at",
"projects_toolresult"."created_at" FROM
"projects_toolresult" WHERE
("projects_toolresult"."status" = 1 AND
"projects_toolresult"."tool_id" = 21)
ORDER BY "projects_toolresult"."updated_at"
DESC LIMIT 1;
And it is weird that when I order query by updated_at field my query executes 60 sec.
Limit (cost=0.43..510.94 rows=1 width=151) (actual
time=56754.932..56754.932 rows=0 loops=1)
-> Index Scan using projects_to_updated_266459_idx on projects_toolresult (cost=0.43..1800549.09 rows=3527 width=151) (actual time=56754.930..56754.930 rows=0 loops=1)
Filter: ((status = 1) AND (tool_id = 21))
Rows Removed by Filter: 13709343 Planning time: 0.236 ms Execution time: 56754.968 ms (6 rows)
No matter if it will be ASC or DESC
But if I do ORDER BY RAND() or without order:
Limit (cost=23496.10..23496.10 rows=1 width=151) (actual time=447.532..447.532 rows=0 loops=1)
-> Sort (cost=23496.10..23505.20 rows=3642 width=151) (actual time=447.530..447.530 rows=0 loops=1)
Sort Key: (random())
Sort Method: quicksort Memory: 25kB
-> Index Scan using projects_toolresult_tool_id_34a3bb16 on projects_toolresult (cost=0.56..23477.89 rows=3642 width=151) (actual time=447.513..447.513 rows=0 loops=1)
Index Cond: (tool_id = 21)
Filter: (status = 1)
Rows Removed by Filter: 6097
Planning time: 0.224 ms
Execution time: 447.571 ms
(10 rows)
It working fast.
I have index on updated_at and status fields(I also tried without too). I did upgrade for default postgres settings, increased values with this generator: https://pgtune.leopard.in.ua/#/
And this is what happens when this queries in action.
Postgres version 9.5
My table and indexes:
id | integer | not null default nextval('projects_toolresult_id_seq'::regclass)
status | smallint | not null
object_id | integer | not null
created_at | timestamp with time zone | not null
content_type_id | integer | not null
tool_id | integer | not null
updated_at | timestamp with time zone | not null
output_data | text | not null
Indexes:
"projects_toolresult_pkey" PRIMARY KEY, btree (id)
"projects_toolresult_content_type_id_object_i_71ee2c2e_uniq" UNIQUE CONSTRAINT, btree (content_type_id, object_id, tool_id)
"projects_to_created_cee389_idx" btree (created_at)
"projects_to_tool_id_ec7856_idx" btree (tool_id, status)
"projects_to_updated_266459_idx" btree (updated_at)
"projects_toolresult_content_type_id_9924d905" btree (content_type_id)
"projects_toolresult_tool_id_34a3bb16" btree (tool_id)
Check constraints:
"projects_toolresult_object_id_check" CHECK (object_id >= 0)
"projects_toolresult_status_check" CHECK (status >= 0)
Foreign-key constraints:
"projects_toolresult_content_type_id_9924d905_fk_django_co" FOREIGN KEY (content_type_id) REFERENCES django_content_type(id) DEFERRABLE INITIALLY DEFERRED
"projects_toolresult_tool_id_34a3bb16_fk_projects_tool_id" FOREIGN KEY (tool_id) REFERENCES projects_tool(id) DEFERRABLE INITIALLY DEFERRED

Answer 1

You are filtering your data on status and tool_id, and sorting on updated_at but you have no single index for all three of those columns.
Add an index, like so:
CREATE INDEX ON projects_toolresult (status, tool_id, updated_at);