[PATCH] extindex: dedupe: reduce SQLite contention and dirty data

[Eric Wong <e@80x24.org>]

Complex queries causes SQLite to block readers for longer than
their retry period.  For dedupe, it was also preventing us from
making good use of checkpoints due to the query time.

With many deduplications, checkpoints are necessary to maintain
system health due to having too much data piled up.
---
 lib/PublicInbox/ExtSearchIdx.pm | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/lib/PublicInbox/ExtSearchIdx.pm b/lib/PublicInbox/ExtSearchIdx.pm
index a421e16b..0e27bba6 100644
--- a/lib/PublicInbox/ExtSearchIdx.pm
+++ b/lib/PublicInbox/ExtSearchIdx.pm
@@ -884,19 +884,20 @@ sub eidx_dedupe ($$) {
 	my $iter;
 	my $min_id = 0;
 	local $sync->{-regen_fmt} = "dedupe %u/".$self->{oidx}->max."\n";
+
+	# note: we could write this query more intelligently,
+	# but that causes lock contention with read-only processes
 dedupe_restart:
 	$iter = $self->{oidx}->dbh->prepare(<<EOS);
-SELECT DISTINCT(mid),id FROM msgid WHERE id IN
-(SELECT id FROM id2num WHERE id > ? GROUP BY num HAVING COUNT(num) > 1)
-ORDER BY id
+SELECT mid,id FROM msgid WHERE id > ? ORDER BY id ASC
 EOS
 	$iter->execute($min_id);
 	while (my ($mid, $id) = $iter->fetchrow_array) {
 		last if $sync->{quit};
 		$self->{current_info} = "dedupe $mid";
 		${$sync->{nr}} = $min_id = $id;
-		my ($n, $prv, @smsg);
-		while (my $x = $self->{oidx}->next_by_mid($mid, \$n, \$prv)) {
+		my ($prv, @smsg);
+		while (my $x = $self->{oidx}->next_by_mid($mid, \$id, \$prv)) {
 			push @smsg, $x;
 		}
 		next if scalar(@smsg) < 2;
@@ -918,8 +919,7 @@ EOS
 		# need to wait on every single one
 		$self->git->async_wait_all;
 
-		# is checkpoint needed? $iter is a very expensive query to restart
-		if (0 && checkpoint_due($sync)) {
+		if (checkpoint_due($sync)) {
 			undef $iter;
 			reindex_checkpoint($self, $sync);
 			goto dedupe_restart;