[OSSTEST PATCH v2 09/41] sg-report-flight: Use WITH to use best index use for $flightsq

[Ian Jackson <ian.jackson@xxxxxxxxxxxxx>]

While we're here, convert this EXISTS subquery to a JOIN.

Perf: runtime of my test case now ~200-300s.

Example query before (from the Perl DBI trace):

      SELECT * FROM (
        SELECT DISTINCT flight, blessing
             FROM flights
             JOIN runvars r1 USING (flight)

            WHERE (branch='xen-unstable')
              AND ( (TRUE AND flight <= 151903) AND (blessing='real') )
                  AND EXISTS (SELECT 1
                            FROM jobs
                           WHERE jobs.flight = flights.flight
                             AND jobs.job = ?)

              AND r1.name LIKE 'built_revision_%'
              AND r1.name = ?
              AND r1.val= ?

            ORDER BY flight DESC
            LIMIT 1000
      ) AS sub
      ORDER BY blessing ASC, flight DESC

With bind variables:

     "test-armhf-armhf-libvirt"
     'built_revision_xen'
     '165f3afbfc3db70fcfdccad07085cde0a03c858b'

After:

      WITH sub AS (
        SELECT DISTINCT flight, blessing
             FROM flights
             JOIN runvars r1 USING (flight)

            WHERE (branch='xen-unstable')
              AND ( (TRUE AND flight <= 151903) AND (blessing='real') )
              AND r1.name LIKE 'built_revision_%'
              AND r1.name = ?
              AND r1.val= ?

            ORDER BY flight DESC
            LIMIT 1000
      )
      SELECT *
        FROM sub
        JOIN jobs USING (flight)

       WHERE (1=1)
                  AND jobs.job = ?

      ORDER BY blessing ASC, flight DESC

With bind variables:

    'built_revision_xen'
    '165f3afbfc3db70fcfdccad07085cde0a03c858b'
    "test-armhf-armhf-libvirt"

Diff to the query:

-      SELECT * FROM (
+      WITH sub AS (
         SELECT DISTINCT flight, blessing
              FROM flights
              JOIN runvars r1 USING (flight)

             WHERE (branch='xen-unstable')
               AND ( (TRUE AND flight <= 151903) AND (blessing='real') )
-                  AND EXISTS (SELECT 1
-                            FROM jobs
-                           WHERE jobs.flight = flights.flight
-                             AND jobs.job = ?)
-
               AND r1.name LIKE 'built_revision_%'
               AND r1.name = ?
               AND r1.val= ?

             ORDER BY flight DESC
             LIMIT 1000
-      ) AS sub
+      )
+      SELECT *
+        FROM sub
+        JOIN jobs USING (flight)
+
+       WHERE (1=1)
+                  AND jobs.job = ?
+
       ORDER BY blessing ASC, flight DESC

Reviewed-by: George Dunlap <George.Dunlap@xxxxxxxxxx>
Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
---
 sg-report-flight | 39 ++++++++++++++++++++++++---------------
 1 file changed, 24 insertions(+), 15 deletions(-)

diff --git a/sg-report-flight b/sg-report-flight
index 10127582..d06be292 100755
--- a/sg-report-flight
+++ b/sg-report-flight
@@ -180,18 +180,6 @@ END
         return undef;
     }
 
-    my @flightsq_params;
-    my $flightsq_jobcond='(1=1)';
-    if (defined $job) {
-       push @flightsq_params, $job;
-       $flightsq_jobcond = <<END;
-                  AND EXISTS (SELECT 1
-                           FROM jobs
-                          WHERE jobs.flight = flights.flight
-                            AND jobs.job = ?)
-END
-    }
-
     # We build a slightly complicated query to find possibly-relevant
     # flights.  A "possibly-relevant" flight is one which the main
     # flight categorisation algorithm below (the loop over $tflight)
@@ -220,6 +208,7 @@ END
     # still execute the full job-specific recursive examination, for
     # each possibly-relevant flight - that's the $tflight loop body.
 
+    my @flightsq_params;
     my $runvars_joins = '';
     my $runvars_conds = '';
     my $ri=0;
@@ -247,18 +236,38 @@ END
       }
     }
 
+    my $flightsq_jobs_join = '';
+    my $flightsq_jobcond = '';
+    if (defined $job) {
+       push @flightsq_params, $job;
+       $flightsq_jobs_join = <<END;
+        JOIN jobs USING (flight)
+END
+       $flightsq_jobcond = <<END;
+                  AND jobs.job = ?
+END
+    }
+
+    # In psql 9.6 this WITH clause makes postgresql do the flights
+    # query first.  This is good because our built revision index finds
+    # relevant flights very quickly.  Without this, postgresql seems
+    # to like to scan the jobs table.
     my $flightsq= <<END;
-      SELECT * FROM (
+      WITH sub AS (
         SELECT DISTINCT flight, blessing
              FROM flights
 $runvars_joins
             WHERE $branches_cond_q
               AND $blessingscond
-$flightsq_jobcond
 $runvars_conds
             ORDER BY flight DESC
             LIMIT 1000
-      ) AS sub
+      )
+      SELECT *
+        FROM sub
+$flightsq_jobs_join
+       WHERE (1=1)
+$flightsq_jobcond
       ORDER BY blessing ASC, flight DESC
 END
     $flightsq= db_prepare($flightsq);
-- 
2.20.1