Proposed code to implement foreign-key-search-schemas option

pt-online-schema-change.diff

diff --git a/bin/pt-online-schema-change b/bin/pt-online-schema-change
index 84063c6..cf2659c 100755
--- a/bin/pt-online-schema-change
+++ b/bin/pt-online-schema-change
@@ -7796,6 +7796,8 @@ sub main {
       $o->set('drop-old-table', 0);
    }

+   my $search_schema = $o->get('foreign-key-search-schemas');
+
    # Explicit --chunk-size disable auto chunk sizing.
    $o->set('chunk-time', 0) if $o->got('chunk-size');

@@ -7863,6 +7865,13 @@ sub main {
       if ( $alter_fk_method eq 'drop_swap' && !$o->get('drop-new-table') ) {
          $o->save_error("--alter-foreign-keys-method=drop_swap does not work with --no-drop-new-table.");
       }
+
+      if ( $search_schema ne 'all'
+           && $search_schema ne 'same')
+      {
+         $o->save_error("Invalid --foreign-key-search-schemas value: $search_schema");
+      }
+
    }

    eval {
@@ -8234,9 +8243,10 @@ sub main {
    }
    else {
       $child_tables = find_child_tables(
-         tbl    => $orig_tbl,
-         Cxn    => $cxn,
-         Quoter => $q,
+         tbl           => $orig_tbl,
+         Cxn           => $cxn,
+         Quoter        => $q,
+         search_schema => $search_schema
       );
       if ( !$child_tables ) {
          if ( $alter_fk_method ) {
@@ -9792,11 +9802,11 @@ sub check_orig_table {

 sub find_child_tables {
    my ( %args ) = @_;
-   my @required_args = qw(tbl Cxn Quoter);
+   my @required_args = qw(tbl Cxn Quoter search_schema);
    foreach my $arg ( @required_args ) {
       die "I need a $arg argument" unless $args{$arg};
    }
-   my ($tbl, $cxn, $q) = @args{@required_args};
+   my ($tbl, $cxn, $q, $search_schema) = @args{@required_args};

    if ( lc($tbl->{tbl_struct}->{engine} || '') eq 'myisam' ) {
       PTDEBUG && _d(q{MyISAM table, not looking for child tables});
@@ -9807,8 +9817,9 @@ sub find_child_tables {

    my $sql = "SELECT table_schema, table_name "
            . "FROM information_schema.key_column_usage "
-           . "WHERE constraint_schema='$tbl->{db}' "
+           . "WHERE referenced_table_schema='$tbl->{db}' "
            . "AND referenced_table_name='$tbl->{tbl}'";
+   $sql .= " AND table_schema='$tbl->{db}'" if $search_schema eq 'same';
    PTDEBUG && _d($sql);
    my $rows = $cxn->dbh()->selectall_arrayref($sql);
    if ( !$rows || !@$rows ) {
@@ -10733,6 +10744,28 @@ tool's built-in functionality if desired.

 =back

+=item --foreign-key-search-schemas
+
+type: string; default: all
+
+This option specifies which schemas are searched for foreign key references to
+the table being changed.
+
+=over
+
+=item all
+
+Search across all schemas for references to the table. This option may lead to
+poor performance if queries against information_schema are slow.
+
+=item same
+
+Restrict the search for foreign keys to the table's schema. The may speed up
+the search for foreign keys but will miss any foreign keys with cross-schema
+references.
+
+=back
+
 =item --ask-pass

 Prompt for a password when connecting to MySQL.