Tweak how logs are stored

Previously, the query for the jobs page was really slow, as it checked the
load_new_guix_revision_job_log_parts table for each job, doing a sequential
scan through the potentially large table.

Adding an index didn't seem to help, as the query planner would belive the
query could return loads of rows, where actually, all that needed checking is
whether a single row existed with a given job_id.

To avoid adding the index to the load_new_guix_revision_job_log_parts table,
and fighting with the query planner, this commit changes the
load_new_guix_revision_job_logs table to include a blank entry for jobs which
are currently being processed. This is inserted at the start of the job, and
then updated at the end to combine and replace all the parts.

This all means that the jobs page should render quickly now.

guix-data-service/jobs/load-new-guix-revision.scm

@@ -96,7 +96,8 @@
     (string-append
      "SELECT "
      (sql-html-escape (get-characters "contents"))
-     " FROM load_new_guix_revision_job_logs WHERE job_id = $1"))
+     " FROM load_new_guix_revision_job_logs"
+     " WHERE job_id = $1 AND contents IS NOT NULL"))
 
   (define parts-query
     (string-append
@@ -113,6 +114,13 @@
        (((contents))
         contents)))))
 
+(define (insert-empty-log-entry conn job-id)
+  (exec-query
+   conn
+   "INSERT INTO load_new_guix_revision_job_logs (job_id, contents) VALUES
+($1, NULL)"
+   (list job-id)))
+
 (define (combine-log-parts! conn job-id)
   (with-postgresql-transaction
    conn
@@ -120,10 +128,13 @@
      (exec-query
       conn
       (string-append
-       "INSERT INTO load_new_guix_revision_job_logs (job_id, contents) "
-       "SELECT job_id, STRING_AGG(contents, '' ORDER BY id ASC) FROM "
+       "UPDATE load_new_guix_revision_job_logs SET contents = "
+       "("
+       "SELECT STRING_AGG(contents, '' ORDER BY id ASC) FROM "
        "load_new_guix_revision_job_log_parts WHERE job_id = $1 "
-       "GROUP BY job_id")
+       "GROUP BY job_id"
+       ")"
+       "WHERE job_id = $1")
       (list job-id))
      (exec-query
       conn
@@ -613,8 +624,6 @@ SELECT
   ),
   EXISTS (
     SELECT 1 FROM load_new_guix_revision_job_logs WHERE job_id = load_new_guix_revision_jobs.id
-    UNION ALL
-    SELECT 1 FROM load_new_guix_revision_job_log_parts WHERE job_id = load_new_guix_revision_jobs.id
   ) AS log_exists
 FROM load_new_guix_revision_jobs
 ORDER BY load_new_guix_revision_jobs.id DESC")
@@ -686,6 +695,7 @@ ORDER BY load_new_guix_revision_jobs.id DESC")
                        (let ((result
                               (with-postgresql-connection
                                (lambda (logging-conn)
+                                 (insert-empty-log-entry logging-conn id)
                                  (let ((logging-port (log-port id logging-conn)))
                                    (set-current-output-port logging-port)
                                    (set-current-error-port logging-port)

sqitch/deploy/change_load_new_guix_revision_job_logs_contents_to_be_nullable.sql

@@ -0,0 +1,7 @@
+-- Deploy guix-data-service:change_load_new_guix_revision_job_logs_contents_to_be_nullable to pg
+
+BEGIN;
+
+ALTER TABLE load_new_guix_revision_job_logs ALTER COLUMN contents DROP NOT NULL;
+
+COMMIT;

sqitch/revert/change_load_new_guix_revision_job_logs_contents_to_be_nullable.sql

@@ -0,0 +1,7 @@
+-- Revert guix-data-service:change_load_new_guix_revision_job_logs_contents_to_be_nullable from pg
+
+BEGIN;
+
+ALTER TABLE load_new_guix_revision_job_logs ALTER contents SET NOT NULL;
+
+COMMIT;

sqitch/sqitch.plan

@@ -14,3 +14,4 @@ license_support 2019-05-13T20:37:40Z Christopher Baines <mail@cbaines.net> # Add
 dates_to_load_new_guix_revision_jobs 2019-06-02T07:39:49Z Christopher Baines <mail@cbaines.net> # Add dates to the load_new_guix_revision_jobs table
 load_new_guix_revision_job_events 2019-06-02T15:44:41Z Christopher Baines <mail@cbaines.net> # Add new table for guix_revision_job_events
 load_new_guix_revision_job_logs 2019-06-21T14:33:09Z chris <chris@phact> # Add load_new_guix_revision_job_logs
+change_load_new_guix_revision_job_logs_contents_to_be_nullable 2019-07-07T20:10:54Z Christopher Baines <mail@cbaines.net> # Change the contents field in the load_new_guix_revision_job_logs table\nto be nullable.\n\nwith '#' will # be ignored, and an empty message aborts the add.  #\nChange to add: # #\nchange_load_new_guix_revision_job_logs_contents_to_be_nullable #\nsqitch/deploy/change_load_new_guix_revision_job_logs_contents_to_be_nullable.sql\nsqitch/revert/change_load_new_guix_revision_job_logs_contents_to_be_nullable.sql\nsqitch/verify/change_load_new_guix_revision_job_logs_contents_to_be_nullable.sql

sqitch/verify/change_load_new_guix_revision_job_logs_contents_to_be_nullable.sql

@@ -0,0 +1,7 @@
+-- Verify guix-data-service:change_load_new_guix_revision_job_logs_contents_to_be_nullable on pg
+
+BEGIN;
+
+-- XXX Add verifications here.
+
+ROLLBACK;