Question

If I have a column, set as primary index, and set as INT.

If I don’t set it as auto increment and just insert random integers which are unique into it, does that slow down future queries compared to autincrementing?

Does it speed things up if I run OPTIMIZE on a table with its primary and only index as INT? (assuming only 2 columns, and second column is just some INT value)

(the main worry is the upper limit on the autoincrement as theres lots of adds and deletes in my table)

Answer 1

If I don’t set it as auto increment and just insert random integers which are unique into it, does that slow it down compared to autincrementing?

In MyISAM it will in fact speed it (marginally).

In InnoDB, this may slow the INSERT operations down due to page splits.

This of course implies that your numbers are really unique.

Does it speed things up if I optimise a table with its primary and only index as INT? (assuming only 2 columns, and second column is just some INT value)

AUTO_INCREMENT and INT may be used together.

OPTIMIZE TABLE will compact you table and indexes, freeing the space left from the deleted rows and page splits. If you had lots of DELETE operations on the table or INSERT out of order (like in your solution with random numbers), this will help.

It will also bring the logical and physical order of the index pages into consistency with each other which will speed up full scans or ranged queries on PK (PK BETWEEN val1 AND val2), but will hardly matter for random seeks.

(the main worry is the upper limit on the autoincrement as theres lots of adds and deletes in my table)

BIGINT UNSIGNED (which can also be used with AUTO_INCREMENT) may hold up values up to 18446744073709551615.