overidx: document why we don't use SQLite WAL

I was wondering about this myself the other day and had to read
up on it.  So make a note of it for future readers.

diff --git a/lib/PublicInbox/OverIdx.pm b/lib/PublicInbox/OverIdx.pm
index c7f45a6c910a91b98ca3500c0b06ed5e4a8c35c5..008a5d1a936fd981786c2a7158aa2f9271e6f187 100644 (file)
--- a/lib/PublicInbox/OverIdx.pm
+++ b/lib/PublicInbox/OverIdx.pm
@@ -23,6 +23,8 @@ sub dbh_new {
        my $dbh = $self->SUPER::dbh_new(1);
 
        # TRUNCATE reduces I/O compared to the default (DELETE)
+       # We do not use WAL since we're optimized for read-only ops,
+       # (and read-only requires SQLite 3.22.0 (2018-01-22)).
        $dbh->do('PRAGMA journal_mode = TRUNCATE');
 
        # 80000 pages (80MiB on SQLite <3.12.0, 320MiB on 3.12.0+)