If there are enough blocks in a key cache to hold blocks of an entire index, or at least the blocks corresponding to its nonleaf nodes, it makes sense to preload the key cache with index blocks before starting to use it. Preloading enables you to put the table index blocks into a key cache buffer in the most efficient way: by reading the index blocks from disk sequentially.
Without preloading, the blocks are still placed into the key cache as needed by queries. Although the blocks stay in the cache, because there are enough buffers for all of them, they are fetched from disk in random order, and not sequentially.
To preload an index into a cache, use the
LOAD INDEX INTO
CACHE
statement. For example, the following
statement preloads nodes (index blocks) of indexes of the
tables t1
and t2
:
mysql> LOAD INDEX INTO CACHE t1, t2 IGNORE LEAVES;
+---------+--------------+----------+----------+
| Table | Op | Msg_type | Msg_text |
+---------+--------------+----------+----------+
| test.t1 | preload_keys | status | OK |
| test.t2 | preload_keys | status | OK |
+---------+--------------+----------+----------+
The IGNORE LEAVES
modifier causes only
blocks for the nonleaf nodes of the index to be preloaded.
Thus, the statement shown preloads all index blocks from
t1
, but only blocks for the nonleaf nodes
from t2
.
If an index has been assigned to a key cache using a
CACHE INDEX
statement,
preloading places index blocks into that cache. Otherwise, the
index is loaded into the default key cache.