Manual:$wgCommentTableSchemaMigrationStage

Details
$wgCommentTableSchemaMigrationStage can be set to one of the Schema change migration flags. The allowed value range for the configuration is one of the  constants. The available constants are as follows:
 * (0, default value): Only read and write the old schema. The new schema need not even exist. This is used from when the patch is merged until the schema change is actually applied to the database.
 * (1): Write both the old and new schema. Read the new schema preferentially, falling back to the old. This is used while the change is being tested, allowing easy roll-back to the old schema.
 * (2): Write only the new schema. Read the new schema preferentially, falling back to the old. This is used while running the maintenance script to migrate existing entries in the old schema to the new schema.
 * (3): Only read and write the new schema. The old schema (and the feature flag) may now be removed.