The creation of the `pg_trgm` extension needs superuser power. So,
this patch makes the extension creation in the Hydra NixOS module when
a local database is used.
If it is not possible to create this extension (remote database for
instance with nosuperuser), the creation of the `pg_trgm` index is
skipped (this index speedup queries on builds.drvpath) and warnings
are emitted:
initialising the Hydra database schema...
WARNING: Can not create extension pg_trgm: permission denied to create extension "pg_trgm"
WARNING: HINT: Temporary provide superuser role to your Hydra Postgresql user and run the script src/sql/upgrade-57.sql
WARNING: The pg_trgm index on builds.drvpath has been skipped (slower complex queries on builds.drvpath)
This allows to keep smooth migrations: the migration process doesn't
require a manual step (but this manual step is recommended on big
remote databases).
The search query uses the LIKE operator which requires a sequential
scan (it can't use the already existing B-tree index). This new
index (trigram) avoids a sequential scan of the builds table when the
LIKE operator is used.
Here is the analyze of a request on the builds table with this index:
explain analyze select * from builds where drvpath like '%k3r71gz0gv16ld8rhcp2bb8gb5w1xc4b%';
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on builds (cost=128.00..132.01 rows=1 width=492) (actual time=0.070..0.077 rows=1 loops=1)
Recheck Cond: (drvpath ~~ '%k3r71gz0gv16ld8rhcp2bb8gb5w1xc4b%'::text)
-> Bitmap Index Scan on indextrgmbuildsondrvpath (cost=0.00..128.00 rows=1 width=0) (actual time=0.047..0.047 rows=3 loops=1)
Index Cond: (drvpath ~~ '%k3r71gz0gv16ld8rhcp2bb8gb5w1xc4b%'::text)
Total runtime: 0.206 ms
(5 rows)
