pgsql/migrations: add ldfv compound index

[jzelinskie]

This speeds up the SearchNotificationLayerIntroducingVulnerability query
by an order magnitude.

After adding this index and vacuuming the table, the following query changes from 570578.327 ms to 1239.984 ms execution time:

EXPLAIN ANALYZE WITH subquery AS (
  SELECT l.ID, l.name
  FROM Vulnerability_Affects_FeatureVersion vafv, FeatureVersion fv, Layer_diff_FeatureVersion ldfv, Layer l
  WHERE l.id >= $2
    AND vafv.vulnerability_id = $1
    AND vafv.featureversion_id = fv.id
    AND ldfv.featureversion_id = fv.id
    AND ldfv.modification = 'add'
    AND ldfv.layer_id = l.id
  ORDER BY l.ID
)
SELECT *
FROM subquery
LIMIT $3;

[Quentin-M]

LGTM.

Actually, I noticed yesterday that the query planner was taking bad decisions such as doing seq scans rather than index scans at few places where the index was present. It must have been due to the table not being properly analyzed, with PostgreSQL considering that the query will return > 5-10% of the total table.

Verified performance gain with the index and analyzed table.