Since before MySQL was MySQL, there has been the .FRM file. Of course, what it really wanted to be was “.form” – a file that stored how to display a form on your (green) CRT. Jan blogged earlier in the year on this still being there, even in MySQL 5.1 (albeit not in any useful form).
So why do we want it to die?
Well… it’s not exactly very useful anymore.
There are a few things it’s used for….
If database/table.frm exists, the table exists (or, on Windows, you may also get databasetable.frm). This is tested in a few bits in the code by a call to access(2).
Most engines have their own data dictionaries (Innodb, PBXT, NDB, Falcon). Keeping these in sync with the FRMs can be problematic at best. This is especially true with distributed engines such as NDB.
The current solution is that on the SQL node that is creating the table, we create the FRM file, gzip it, and store it in the cluster. Then, other nodes, if they go “err… no local frm” first call ha_create_table_from_engine() which NDB will go and see if the table exists in the cluster. If so, it copies the FRM from the cluster to local disk and then the SQL server continues on its way with the standard way of opening a table (through the FRM). If you do DDL through the NDB-API (and not via SQL) then well… you get to keep both pieces.
As for if you crash during a table rename (with any engine with its own data dictionary.. e.g. InnoDB)… you again get to keep both pieces. (There is a bit of discussion on this over here)
Having FRM files also doesn’t especially lead to having multiple versions of table metadata co-existing in the server.
The fun part of reading a frm is open_binary_frm in table.cc. It reads in the frm into a TABLE_SHARE. If we only had some other way of filling out a TABLE_SHARE… one from the engine itself…
But what about any metadata that the engine data dictionary doesn’t have? For example, many server types may map to 1 engine type. An example of this is the GIS types in MySQL. For most engines, these just map to BLOBs. The engine itself has no knowledge about that, but we should fill out the table definition correctly…. so for this type of thing the engine may need to store some additional metadata. This is pretty easy for transactional engines: put it in a table! (although you then have your own problem about keeping this synchronised with any DDL). For engines that don’t have their own data dictionary, we can just provide a set of routines to store/read a frm type file (based on protobufs no doubt).
There also seems to be some entanglement with LOCK_open. Ahhh LOCK_open, the lock that nobody can possibly understand.
The tricky thing will be not rewriting every little bit from scratch all at once but rather go for the incremental bits….