In the previous post I mentioned a way I use to preload Clustered Index (data) for Innodb tables. Though I thought this topic would benefit from a bit more information.
But lest first start with feature request for Innodb Team: All ways I mention here are hacks and they can’t be as efficient as native support. It would be great if Innodb would implement command to preload table to Innodb buffer pool, which would simply go through .ibd file sequentially and inject pages in the buffer pool. This would make preload done using sequential file scan even if indexed suffered a lot of page splits.
Now lets continue to the hacks 🙂
So As I mentioned you can load Innodb Table Clustered Index in the buffer pool pretty efficiently by using something like SELECT count(*) FROM tbl WHERE non_index_col=0 This works relatively well (though can be slow for fragmented tables) but it does not preload indexes in memory neither it does externally stored objects – BLOB and TEXT fields.
If you would like some non PRIMARY Indexes preloaded you can use something like SELECT count(*) from tbl WHERE index_col like “%0%” for each index. Only one such query per index is enough even if it is multiple column index.
To fetch BLOB/TEXT externally stored columns you can use similar query: SELECT count(*) from tbl WHERE blob_col like “%0%”. Note if you preloading BLOB/TEXT columns you do not need to use first query I mentioned because scanning potentially externally stored blobs will also scan Clustered key Anyway.
Now, say you have bunch of tables having few indexes – should you run multiple queries in parallel to get best preload speed ?
It depends – depending on key/clustered key fragmentation it may be faster to run queries one by one (keeping IO more sequential) or run multiple queries at once to get more outstanding requests at the same time – benchmark to find out.
If you just need to preload single large table you can chop it into several ranges and preload in parallel, such as SELECT count(*) FROM tbl WHERE id BETWEEN 1 and 10000000 AND non_index_col=0