MySQL Reference Manual :: 7.2.5.2 Range Access Method for Multiple-Part Indexes

For HASH indexes, each interval containing identical values can be used. This means that the interval can be produced only for conditions in the following form:

    key_part1 cmp const1
AND key_part2 cmp const2
AND ...
AND key_partN cmp constN;

Here, const1, const2, ... are constants, cmp is one of the =, <=>, or IS NULL comparison operators, and the conditions cover all index parts. (That is, there are N conditions, one for each part of an N-part index.)

See Section 7.2.5.1, “Range Access Method for Single-Part Indexes” for the definition of what is considered to be a constant.

For example, the following is a range condition for a three-part HASH index:

key_part1 = 1 AND key_part2 IS NULL AND key_part3 = 'foo'

For a BTREE index, an interval might be usable for conditions combined with AND, where each condition compares a key part with a constant value using =, <=>, IS NULL, >, <, >=, <=, !=, <>, BETWEEN, or LIKE 'pattern' (where 'pattern' doesn't start with a wildcard). An interval can be used as long as it is possible to determine a single key tuple containing all records that match the condition (or two intervals if <> or != is used). For example, for this condition:

key_part1 = 'foo' AND key_part2 >= 10 AND key_part3 > 10

The single interval is:

('foo', 10, 10)
   < (key_part1, key_part2, key_part3)
      < ('foo', +inf, +inf)

It is possible that the created interval contains more records than the initial condition. For example, the preceding interval includes the value ('foo', 11, 0), which does not satisfy the original condition.

If conditions that cover sets of records contained within intervals are combined with OR, they form a condition that covers a set of records contained within the union of their intervals. If the conditions are combined with AND, they form a condition that covers a set of records contained within the intersection of their intervals. For example, for this condition on a two-part index:

(key_part1 = 1 AND key_part2 < 2)
OR (key_part1 > 5)

The intervals is:

(1, -inf) < (key_part1, key_part2) < (1, 2)
(5, -inf) < (key_part1, key_part2)

In this example, the interval on the first line uses one key part for the left bound and two key parts for the right bound. The interval on the second line uses only one key part. The key_len column in the EXPLAIN output indicates the maximum length of the key prefix used.

In some cases, key_len may indicate that a key part was used, but that might be not what you would expect. Suppose that key_part1 and key_part2 can be NULL. Then the key_len column displays two key part lengths for the following condition:

key_part1 >= 1 AND key_part2 < 2

But in fact, the condition is converted to this:

key_part1 >= 1 AND key_part2 IS NOT NULL