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In MySQL, for InnoDB engine, how do I find out the size of one of the indexes I have in the same table ?

I know I can select the index_length from information_schema.tables, but this gives the total size of all indexes in the table.

Let's suppose I have a table with PK index, several FK indexes and unique constraints, etc. Can I find out the size of one of those indexes only? Let's say, I'd like to know the size of the PK index only.

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migrated from Feb 6 '12 at 19:18

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innodb or myiasm? – BCoates Mar 7 '12 at 21:48
InnoDB. I've also update the original question with this, thanks. – Timur Mar 8 '12 at 8:56
up vote 4 down vote accepted

There appears to be no way to get the answer out of mysql directly; however you can do a pretty good job of estimating it:

For innodb, PRIMARY index is part of the data itself and shows up in data_length. If a table has no other indexes index_length is 0.

For every row in your table, every non-PRIMARY (UNIQUE or INDEX) index stores

  • 6 bytes overhead
  • every column indexed
  • every column in PRIMARY not already in the index
  • 1 bit per possible NULL column, rounded up to the byte

The size of a column in an index is the space the actual data takes up plus 1 or 2 bytes of length per VARCHAR/TEXT/BLOB. NULLs take no space.

There is an additional overhead of about 1.4x to 2.8x for the B-tree pages and non-full leaf pages. In my databases it looks to be around 2x usually.

FKs should not have any space overhead beyond the index they require.

mysql docs - source

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Thanks, this is great! This is very detailed indeed. – Timur Mar 9 '12 at 8:58

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