History@@join_cache_level variable controls whether BKA is used. Its values are documented here: http://askmonty.org/wiki/Manual:Batched_Key_Access .
| @join_cache_level value | Incremental buffering | Join algorithm |
|---|---|---|
| 0 | No | No join buffering |
| 1 | No | Block Nested Loop |
| 2 | Yes | Block Nested Loop |
| 3 | No | Block Nested Loop Hashed |
| 4 | Yes | Block Nested Loop Hashed |
| 5 | No | Batched Key Acccess |
| 6 | Yes | Batched Key Access |
| 7 | No | Batched Key Access, Hashed |
| 8 | Yes | Batched Key Access, Hashed |
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