Create a pg_trgm index on builds.drvpath

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)

src/sql/hydra.sql

@@ -688,3 +688,9 @@ create index IndexBuildsOnKeep on Builds(keep) where keep = 1;
 create index IndexJobsetEvalsOnJobsetId on JobsetEvals(project, jobset, id desc) where hasNewBuilds = 1;
 
 create index IndexBuildsOnNotificationPendingSince on Builds(notificationPendingSince) where notificationPendingSince is not null;
+
+#ifdef POSTGRESQL
+-- Provide an index used by LIKE operator on builds.drvpath (search query)
+CREATE EXTENSION pg_trgm;
+CREATE INDEX IndexTrgmBuildsOnDrvpath ON builds USING gin (drvpath gin_trgm_ops);
+#endif

src/sql/upgrade-57.sql

@@ -0,0 +1,2 @@
+CREATE EXTENSION pg_trgm;
+CREATE INDEX IndexTrgmBuildsOnDrvpath ON builds USING gin (drvpath gin_trgm_ops);