Constrain environment deployments to project IDs
When querying the deployments of an environment the query Rails produces
will be along the lines of the following:
SELECT *
FROM deployments
WHERE environment_id = X
For queries such as this (or queries that use this as their base and add
more conditions) there is no meaningful index that can be used as long
as deployments.project_id is not part of a WHERE clause.
To work around this we change that "has_many :deployments" relation to
always add a "WHERE project_id = X" condition. This means that queries
filtering deployments can make better use of the existing indexes. For
example, when filtering by deployments.iid this will result in the
following query:
SELECT *
FROM deployments
WHERE environment_id = X
AND project_id = Y
AND iid = Z
This means PostgreSQL can use the existing index on
(project_id, environment_id, iid) instead of having to use a different
index (or none at all) and having to scan over a large amount of data.
Query plan wise this means that instead of this query and plan:
EXPLAIN (BUFFERS, ANALYZE)
SELECT deployments.*
FROM deployments
WHERE deployments.environment_id = 5
AND deployments.iid = 225;
Index Scan using index_deployments_on_project_id_and_iid on deployments (cost=0.42..14465.75 rows=1 width=117) (actual time=6.394..38.048 rows=1 loops=1)
Index Cond: (iid = 225)
Filter: (environment_id = 5)
Rows Removed by Filter: 839
Buffers: shared hit=4534
Planning time: 0.076 ms
Execution time: 38.073 ms
We produce the following query and plan:
EXPLAIN (BUFFERS, ANALYZE)
SELECT deployments.*
FROM deployments
WHERE deployments.environment_id = 5
AND deployments.iid = 225
AND deployments.project_id = 1292351;
Index Scan using index_deployments_on_project_id_and_iid on deployments (cost=0.42..4.45 rows=1 width=117) (actual time=0.018..0.018 rows=1 loops=1)
Index Cond: ((project_id = 1292351) AND (iid = 225))
Filter: (environment_id = 5)
Buffers: shared hit=4
Planning time: 0.088 ms
Execution time: 0.039 ms
On GitLab.com these changes result in a (roughly) 11x improvement in SQL
timings for the CI environment status endpoint.
Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/36877
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