overidx: document the SQLite PRAGMA we use

This ought to prevent cargo-culting the cache_size PRAGMA
into smaller SQLite DBs we might use.

diff --git a/lib/PublicInbox/OverIdx.pm b/lib/PublicInbox/OverIdx.pm
index acbf2c8de6092742e10cdda77415812cbf6778d6..cb15baadf2b8ef525ea8fc2ef7857f5878087ab3 100644 (file)
--- a/lib/PublicInbox/OverIdx.pm
+++ b/lib/PublicInbox/OverIdx.pm
@@ -21,8 +21,16 @@ use PublicInbox::Search;
 sub dbh_new {
        my ($self) = @_;
        my $dbh = $self->SUPER::dbh_new(1);
+
+       # TRUNCATE reduces I/O compared to the default (DELETE)
        $dbh->do('PRAGMA journal_mode = TRUNCATE');
+
+       # 80000 pages (80MiB on SQLite <3.12.0, 320MiB on 3.12.0+)
+       # was found to be good in 2018 during the large LKML import
+       # at the time.  This ought to be configurable based on HW
+       # and inbox size; I suspect it's overkill for many inboxes.
        $dbh->do('PRAGMA cache_size = 80000');
+
        create_tables($dbh);
        $dbh;
 }