diff --git a/scripts/Modules/Mars.pm b/scripts/Modules/Mars.pm
index 38ce83f..922a6c7 100644
--- a/scripts/Modules/Mars.pm
+++ b/scripts/Modules/Mars.pm
@@ -166,6 +166,18 @@ sub db_do {
     $db->do(@cmd) or die "Database command failed (@cmd): " . $db->errstr;
 }
 
+# db_do only reports affected rows, this one also reads the value back
+sub db_selectrow {
+    my (@cmd) = @_;
+
+    my $db = db_connect();
+
+    my @row = $db->selectrow_array(@cmd);
+    die "Database query failed (@cmd): " . $db->errstr if $db->err;
+
+    return @row;
+}
+
 sub db_create {
     my ($db_name) = @_;
     say " - Creating database $db_name...";
diff --git a/scripts/menu.pl b/scripts/menu.pl
index d45d101..d219a98 100644
--- a/scripts/menu.pl
+++ b/scripts/menu.pl
@@ -848,6 +848,136 @@ my %handlers = (
             '<', Util::shell_quote($dump_file) );
     },
 
+    '33. Repair fspgroup Write Groups (Mars)' => sub {
+
+        # Retroactively fixes (hopefully) the missing equivalence class
+        # mappings (EC <-> Pilot) the BasicPruner misses (because it maps
+        # multiple classes to a single pilot, but uses the pilot as the
+        # primary key)
+        my @dbs = Mars::db_list();
+        my @dbs_with_notes;
+        foreach my $db (@dbs) {
+            my $info =
+              Util::read_experiment_info( $db =~ s/smchurla_//r =~ s/:/-/gr );
+
+            push @dbs_with_notes,
+              ( defined $info && length($info) > 0 )
+              ? sprintf( "%-60s (%s)", $db, $info )
+              : $db;
+        }
+
+        my @selected_dbs = TUI::select_from_list( "Select Databases to Repair",
+            1, @dbs_with_notes );
+        die "No database selected" unless @selected_dbs;
+
+        @selected_dbs =
+          map { s/(.*?)\s+\(.+\)$/$1/r } @selected_dbs;
+
+        # The PRIMARY KEY gets dropped by the repair and replaced with a KEY.
+        # To not run this shit on already repaired DBs, check for the PRIMARY
+        # index as a determinant
+        my $index_exists = sub {
+            my ($index) = @_;
+            my ($count) = Mars::db_selectrow(
+                "SELECT COUNT(*) FROM information_schema.STATISTICS
+                 WHERE TABLE_SCHEMA = DATABASE()
+                   AND TABLE_NAME = 'fspgroup'
+                   AND INDEX_NAME = '$index'"
+            );
+            return $count;
+        };
+
+        foreach my $db (@selected_dbs) {
+            say "Repairing $db...";
+
+            # Select the DB so index_exists works
+            Mars::db_do("use `$db`");
+
+            # Update the variant names, so the queries still work
+            my $expected_variant = $db =~ s/^smchurla_//r;
+            my ($variant_count) =
+              Mars::db_selectrow("SELECT COUNT(DISTINCT variant) FROM variant");
+            my ($current_variant) =
+              Mars::db_selectrow("SELECT DISTINCT variant FROM variant");
+
+            if ( !defined $variant_count || $variant_count != 1 ) {
+                say " - WARNING: $variant_count distinct variants!";
+            }
+            elsif ( $current_variant ne $expected_variant ) {
+                say " - Variant is '$current_variant' but queries look up"
+                  . " '$expected_variant' (from the database name), renaming...";
+                Mars::db_do("UPDATE variant SET variant = '$expected_variant'");
+            }
+
+            # Drop the primary key
+            if ( $index_exists->('PRIMARY') ) {
+                say " - Dropping PRIMARY KEY (pilot_id)...";
+                Mars::db_do("ALTER TABLE fspgroup DROP PRIMARY KEY");
+            }
+
+            # Add an index for the columns we actually join on (mostly)
+            unless ( $index_exists->('eq_class') ) {
+                say " - Adding eq_class index...";
+                Mars::db_do(
+                    "ALTER TABLE fspgroup
+ADD KEY eq_class (variant_id, instr2, data_physical_address)"
+                );
+            }
+
+            # I'm currently only using BasicPruner, but don't touch other
+            # pruning methods for now, for safety
+            my ($fspmethod_id) =
+              Mars::db_selectrow(
+                "SELECT id FROM fspmethod WHERE method = 'basic'");
+            die "No 'basic' fspmethod in $db" unless defined $fspmethod_id;
+
+            # The first row succeeded before the duplicate key error, remove it
+            Mars::db_do(
+                "DELETE g
+FROM fspgroup g
+JOIN fsppilot p ON p.id = g.pilot_id
+WHERE p.known_outcome = 1 AND p.fspmethod_id = $fspmethod_id"
+            );
+
+            my ($expected) =
+              Mars::db_selectrow(
+                "SELECT COUNT(*) FROM trace WHERE accesstype = 'W'");
+
+            # Query from BasicPruner.cc, now ran against the updated DB
+            say " - Inserting $expected write groups...";
+            Mars::db_do(
+                "INSERT INTO
+fspgroup (variant_id, instr2, data_physical_address, bit_pos, fspmethod_id, pilot_id)
+SELECT STRAIGHT_JOIN t.variant_id, t.instr2, t.data_physical_address, p.bit_pos, p.fspmethod_id, p.id
+FROM fsppilot p
+JOIN trace t
+ON t.variant_id = p.variant_id AND p.fspmethod_id = $fspmethod_id AND p.known_outcome = 1
+WHERE t.accesstype = 'W'"
+            );
+
+            # We expect one row per write group/EC.
+            # Otherwise no idea what's going on :O
+            my ($actual) = Mars::db_selectrow(
+                "SELECT COUNT(*)
+FROM fspgroup g
+JOIN fsppilot p ON p.id = g.pilot_id
+WHERE p.known_outcome = 1 AND p.fspmethod_id = $fspmethod_id"
+            );
+
+            if ( $actual == $expected ) {
+                say " - OK: $actual write groups match the write ECs in trace";
+            }
+            else {
+                say " - WARNING: inserted $actual, expected $expected."
+                  . " Check for duplicate known_outcome pilots:"
+                  . " SELECT variant_id, COUNT(*) FROM fsppilot"
+                  . " WHERE known_outcome = 1 GROUP BY variant_id;";
+            }
+        }
+
+        say "Queries have to be re-run.";
+    },
+
     '95. Delete Builds' => sub {
 
         # Delete old build files