Create index and then insert or insert and then create index?

Question

I'm inserting a big volume of data in a table in Mysql, I need to create an index to access quickly to the data, however, I would like to know if there is a difference (in performance) between these scenarios:

• Create an index and then insert all data
• Insert all data and then create an index

thanks in advance!

Answer 1

For InnoDB storage engine, for the cluster index, it will be faster to specify the cluster index (ie PRIMARY KEY ) on the table before inserting data.

This is because if a cluster index (PRIMARY KEY) is not defined on the table, then InnoDB will use a hidden 6-byte auto-incremented counter for the cluster index. If a PRIMARY KEY is later specified, the entire table will need to be rebuilt.

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For secondary indexes (ie non-cluster indexes) with InnoDB, it is usually faster to insert data without secondary indexes defined, and then build the secondary indexes after the data is loaded.

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FOLLOWUP

As far as the speed of loading to a table (in particular, a table that is truncated/emptied, and then reloaded), dropping and re-creating indexes is a well known technique for speeding up processing, not just with MySQL, but with other RDBMS such as Oracle.)

There isn't a guarantee that the processing will be faster; as with most things database, we need tests to determine which is faster.

For a table containing millions of rows, and we're adding a couple dozen hundred rows, then dropping and rebuilding indexes is likely going to be a lot slower, because of all of the extra work to re-index all of the existing rows. It would be faster to do the index maintenance while the rows are being inserted.

In terms of speeding up a load, the "drop and recreate indexes" technique isn't going to give us the kind of dramatic improvements we get from other changes. For example, it won't be anywhere near the improvement we would see by using LOAD DATA in place of INSERT statements, nor using multi-row INSERT statements vs a series of singleton INSERT statements.