Question

Assume I have this table:

create table table_a (
    id int,
    name varchar(25),
    address varchar(25),
    primary key (id)
) engine = innodb;

When I run this query:

select * from table_a where id >= 'x' and name = 'test';

How will MySQL process it? Will it pull all the id’s first (assume 1000 rows) then apply the where clause name = ‘test’?

Or while it looks for the ids, it is already applying the where clause at the same time?

Answer 1

As id is the PK (and no index on name) it will load all rows that satisfy the id based criterion into memory after which it will filter the resultset by the name criterion. Adding a composite index containing both fields would mean that it would only load the records that satisfy both criteria. Adding a separate single column index on the name field may not result in an index merge operation, in which case the index would have no effect.