how does MySQL know which partition to look up?

Question

Let's analyse the simplest possible example of MySQL paritioning by hash (slightly modified version of http://dev.mysql.com/doc/refman/5.5/en/alter-table-partition-operations.html):

CREATE TABLE t1 (
    id INT,
    year_col INT
);

ALTER TABLE t1
    PARTITION BY HASH(year_col)
    PARTITIONS 8;

Let's say we put there millions of records. The question is - if a specific query comes (e.g. SELECT * FROM t1 WHERE year_col = 5) then how does MySQL know which partition to look up? There are 8 partitions. I guess that the hash function is calculated and MySQL recognizes that it matches thepartitioning key and then MySQL knows which one that is. But what is the query is SELECT * FROM t1 WHERE year_col IN (5, 45, 5435)? How about other non-trivial queries? Is there any general algorithm for that?

Answer 1

This is called Partition pruning:

The optimizer can perform pruning whenever a WHERE condition can be reduced to either one of the following two cases:

• partition_column = constant

• partition_column IN (constant1, constant2, ..., constantN)

In the first case, the optimizer simply evaluates the partitioning expression for the value given, determines which partition contains that value, and scans only this partition. (...)

In the second case, the optimizer evaluates the partitioning expression for each value in the list, creates a list of matching partitions, and then scans only the partitions in this partition list. (...)

MySQL can apply partition pruning to SELECT, DELETE, and UPDATE statements. INSERT statements currently cannot be pruned.

Pruning can also be applied to short ranges, which the optimizer can convert into equivalent lists of values. (...)