Add ability to ignore tables in the schema cache #43075
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In rails#43075 I added the ability to skip adding a configured set of tables to the schema cache. After pushing that PR I found the behavior across methods and database adapters to be inconsistent when a table didn't exist. Since ignored tables should behave the same as missing tables, I wanted to make the behavior consistent. Previously the behavior was: *mysql* columns & columns_hash: Raised db error for non-existent table primary_keys: returned `nil` for non-existent table indexes: Raised db error for non-existent table *sqlite3* columns & columns_hash: Raised db error for non-existent table primary_keys: returned `nil` for non-existent table indexes: returned `[]` for non-existent table *postgresql* columns & columns_hash: Raised db error for non-existent table primary_keys: returned `nil` for non-existent table indexes: returned `[]` for non-existent table Current behavior: *mysql, sqlite3, and postgresql* columns & columns_hash: Raise db error for non-existent table primary_keys: returns `nil` for non-existent table indexes: returns `[]` for non-existent table The only change is to mysql, which used to raise for indexes, now it returns and empty array like all the other adapters.
In cases where an application uses pt-osc or lhm they may have temporary tables being used for migrations. Those tables shouldn't be included by the schema cache because it makes the cache bigger and those tables shouldn't ever be queried by the application. This feature allows applications to configure a list of or regex of tables to ignore. The behavior for ignored tables for each method in the schema cache (`columns`, `columns_hash`, `primary_keys`, and `indexes`) matches the behavior of a non-existent table. `columns` and `columns_hash` will raise a database not found error, `primary_keys` will return `nil`. and `indexes` will return an empty array. See rails#43105 which make behavior across adapters consistent. To use in an application configure `config.active_record.schema_cache_ignored_tables` to an array of tables or regex's. Those tables will not be included in the schema cache dump.
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In cases where an application uses pt-osc or lhm they may have
temporary tables being used for migrations. Those tables shouldn't be
included by the schema cache because it makes the cache bigger and those
tables shouldn't ever be queried by the application. This feature allows
applications to configure a list of or regex of tables to ignore. The
behavior for ignored tables for each method in the schema cache
(
columns,columns_hash,primary_keys, andindexes) matches thebehavior of a non-existent table.
To use in an application configure
config.active_record.schema_cache_ignored_tablesto an array of tables or regex's. Those tables will not be included in
the schema cache dump.
cc/ @rafaelfranca @casperisfine @tenderlove - I think this will work for your schema cache. Note that in our app I found we actually weren't ensuring that
primary_keys,columns,columns_hashandindexeswere behaving the same as a non-existent table which meant that if these tables were ever queried then they'd populate the cache in-memory. Since these tables are not supposed to be queried this technically shouldn't change behavior when your app opts into this.