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Is there a way to determine whether a mysql index fits entirely in available memory? If so, how would I:
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An index does not need to be kept in memory. It acts just like a table -- it is composed of 16KB blocks that are cached into the buffer pool as needed, then bumped out when 'old' (think "least-recently-used" caching schemes).
An index is usually maintained as a B+ Tree on disk & in-memory, and any index is stored in blocks on disk. These blocks are called index blocks. The entries in the index block are always sorted on the index/search key.
One Index size: 3 bytes for DATE. 7 bytes for DATETIME(3)
If there is a choice between multiple indexes, MySQL normally uses the index that finds the smallest number of rows (the most selective index). If the table has a multiple-column index, any leftmost prefix of the index can be used by the optimizer to look up rows.
Depends on Storage Engine
SELECT FLOOR(SUM(index_length)/POWER(1024,2)) IndexSizesMB FROM information_schema.tables WHERE engine='MyISAM' AND table_schema NOT IN ('information_schema','performance_schema','mysql'); Subtract that from key_buffer_size. If the answer > 0, then Yes
SELECT FLOOR(SUM(data_length+index_length)/POWER(1024,2)) InnoDBSizeMB FROM information_schema.tables WHERE engine='InnoDB'; Subtract that from innodb_buffer_pool_size. If the answer > 0, then Yes
I wrote about this in the DBA StackExchange
On a dedicated DB Server, make sure InnoDBSizeMB+IndexSizesMB does not exceed 75% of RAM.
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