lib/PublicInbox/Over.pm    | 19 +++++++++++++++++++
 lib/PublicInbox/OverIdx.pm | 60 ++++++++++++++++++++++++++++++++---------------------
 t/over.t                   | 27 ++++++++++++++++++++-------

diff --git a/lib/PublicInbox/Over.pm b/lib/PublicInbox/Over.pm
index 081123868795fd9b22997488331b0bc39dc51800..f34e7fc1cb23e10630ea3ab9334a1b4b47e61824 100644
--- a/lib/PublicInbox/Over.pm
+++ b/lib/PublicInbox/Over.pm
@@ -260,6 +260,25 @@ 	my $smsg = $sth->fetchrow_hashref;
 	$smsg ? load_from_row($smsg) : undef;
 }
 
+sub get_xref3 {
+	my ($self, $num) = @_;
+	my $dbh = dbh($self);
+	my $sth = $dbh->prepare_cached(<<'', undef, 1);
+SELECT ibx_id,xnum,oidbin FROM xref3 WHERE docid = ? ORDER BY ibx_id ASC
+
+	$sth->execute($num);
+	my $rows = $sth->fetchall_arrayref;
+	my $eidx_key_sth = $dbh->prepare_cached(<<'', undef, 1);
+SELECT eidx_key FROM inboxes WHERE ibx_id = ?
+
+	[ map {
+		my $r = $_;
+		$eidx_key_sth->execute($r->[0]);
+		my $eidx_key = $eidx_key_sth->fetchrow_array;
+		"$eidx_key:$r->[1]:".unpack('H*', $r->[2]);
+	} @$rows ];
+}
+
 sub next_by_mid {
 	my ($self, $mid, $id, $prev) = @_;
 	my $dbh = dbh($self);
diff --git a/lib/PublicInbox/OverIdx.pm b/lib/PublicInbox/OverIdx.pm
index 09bca790f20a719925e3437072bf1fbb3513f8e4..dff2780d861f71326251744b383cebf9e10304ae 100644
--- a/lib/PublicInbox/OverIdx.pm
+++ b/lib/PublicInbox/OverIdx.pm
@@ -517,19 +517,26 @@ sub eidx_prep ($) {
 	my ($self) = @_;
 	$self->{-eidx_prep} //= do {
 		my $dbh = $self->dbh;
-		$dbh->do(<<'');
-INSERT OR IGNORE INTO counter (key) VALUES ('oidmap_num')
+		$dbh->do(<<"");
+INSERT OR IGNORE INTO counter (key) VALUES ('eidx_docid')
 
 		$dbh->do(<<'');
-INSERT OR IGNORE INTO counter (key) VALUES ('eidx_docid')
+CREATE TABLE IF NOT EXISTS inboxes (
+	ibx_id INTEGER PRIMARY KEY AUTOINCREMENT,
+	eidx_key VARCHAR(255) NOT NULL, /* {newsgroup} // {inboxdir} */
+	UNIQUE (eidx_key)
+)
 
 		$dbh->do(<<'');
-CREATE TABLE IF NOT EXISTS oidmap (
-	num INTEGER NOT NULL, /* NNTP article number == IMAP UID */
-	oidbin VARBINARY, /* 20-byte SHA-1 or 32-byte SHA-256 */
-	UNIQUE (num),
-	UNIQUE (oidbin)
+CREATE TABLE IF NOT EXISTS xref3 (
+	docid INTEGER NOT NULL, /* <=> over.num */
+	ibx_id INTEGER NOT NULL, /* <=> inboxes.ibx_id */
+	xnum INTEGER NOT NULL, /* NNTP article number in ibx */
+	oidbin VARBINARY NOT NULL, /* 20-byte SHA-1 or 32-byte SHA-256 */
+	UNIQUE (docid, ibx_id, xnum, oidbin)
 )
+
+	$dbh->do('CREATE INDEX IF NOT EXISTS idx_docid ON xref3 (docid)');
 
 		$dbh->do(<<'');
 CREATE TABLE IF NOT EXISTS eidx_meta (
@@ -564,28 +571,33 @@ 	my ($self) = @_;
 	get_counter($self->{dbh}, 'eidx_docid');
 }
 
-sub oid2num {
-	my ($self, $oidhex) = @_;
-	my $dbh = eidx_prep($self);
-	my $sth = $dbh->prepare_cached(<<'', undef, 1);
-SELECT num FROM oidmap WHERE oidbin = ?
+sub add_xref3 {
+	my ($self, $docid, $xnum, $oidhex, $eidx_key) = @_;
+	begin_lazy($self);
+	my $ibx_id = id_for($self, 'inboxes', 'ibx_id', eidx_key => $eidx_key);
+	my $oidbin = pack('H*', $oidhex);
+	my $sth = $self->{dbh}->prepare_cached(<<'');
+INSERT OR IGNORE INTO xref3 (docid, ibx_id, xnum, oidbin) VALUES (?, ?, ?, ?)
 
-	$sth->bind_param(1, pack('H*', $oidhex), SQL_BLOB);
+	$sth->bind_param(1, $docid);
+	$sth->bind_param(2, $ibx_id);
+	$sth->bind_param(3, $xnum);
+	$sth->bind_param(4, $oidbin, SQL_BLOB);
 	$sth->execute;
-	$sth->fetchrow_array;
 }
 
-sub oid_add {
-	my ($self, $oidhex) = @_;
-	my $dbh = eidx_prep($self);
-	my $num = adj_counter($self, 'oidmap_num', '+');
-	my $sth = $dbh->prepare_cached(<<'');
-INSERT INTO oidmap (num, oidbin) VALUES (?,?)
+sub remove_xref3 {
+	my ($self, $docid, $oidhex, $eidx_key) = @_;
+	begin_lazy($self);
+	my $ibx_id = id_for($self, 'inboxes', 'ibx_id', eidx_key => $eidx_key);
+	my $oidbin = pack('H*', $oidhex);
+	my $sth = $self->{dbh}->prepare_cached(<<'');
+DELETE FROM xref3 WHERE docid = ? AND ibx_id = ? AND oidbin = ?
 
-	$sth->bind_param(1, $num);
-	$sth->bind_param(2, pack('H*', $oidhex), SQL_BLOB);
+	$sth->bind_param(1, $docid);
+	$sth->bind_param(2, $ibx_id);
+	$sth->bind_param(3, $oidbin, SQL_BLOB);
 	$sth->execute;
-	$num;
 }
 
 1;
diff --git a/t/over.t b/t/over.t
index 3e2860f8f96d0d40805b72f5b954b79e5970c67b..56c20d01799ce7a244eaa82fc3abb13b524efebe 100644
--- a/t/over.t
+++ b/t/over.t
@@ -75,14 +75,27 @@ 		'WAL journal_mode not clobbered if manually set');
 }
 
 # ext index additions
+$over->eidx_prep;
 {
-	my $hex = 'deadbeefcafe';
-	my $n = $over->oid_add($hex);
-	ok($n > 0, 'oid_add returned number');
-	is($over->oid2num($hex), $n, 'oid2num works');
-	my $n2 = $over->oid_add($hex.$hex);
-	ok($n2 > $n, 'oid_add increments');
-	is($over->oid2num($hex.$hex), $n2, 'oid2num works again');
+	my @arg = qw(1349 2019 adeadba7cafe example.key);
+	ok($over->add_xref3(@arg), 'first add');
+	ok($over->add_xref3(@arg), 'add idempotent');
+	my $xref3 = $over->get_xref3(1349);
+	is_deeply($xref3, [ 'example.key:2019:adeadba7cafe' ], 'xref3 works');
+
+	@arg = qw(1349 2018 deadbeefcafe example.kee);
+	ok($over->add_xref3(@arg), 'add another xref3');
+	$xref3 = $over->get_xref3(1349);
+	is_deeply($xref3, [ 'example.key:2019:adeadba7cafe',
+			'example.kee:2018:deadbeefcafe' ],
+			'xref3 works forw two');
+
+	@arg = qw(1349 adeadba7cafe example.key);
+	ok($over->remove_xref3(@arg), 'remove first');
+	$xref3 = $over->get_xref3(1349);
+	is_deeply($xref3, [ 'example.kee:2018:deadbeefcafe' ],
+		'confirm removal successful');
+	$over->rollback_lazy;
 }
 
 done_testing();