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Antoine Eiche 7935cffd6b 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)

2019-06-06 14:28:25 +02:00