Question

Does the following query use full table scan?

If so, is there a way to avoid the full table scan?

  SELECT a.title, 
         COUNT(*) AS `count`
    FROM b
    JOIN a ON a.id = b.a_id
GROUP BY b.a_id

Please note that following indexes exist:

a PRIMARY id

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b PRIMARY c_id THEN a_id

b INDEX a_id

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Here are the results of explain:

id  select_type  table  type    possible_keys  key      ref         rows  extra
--------------------------------------------------------------------------------------
1   SIMPLE       b      index   a_id           a_id     NULL        7     Using index
1   SIMPLE       a      eq_ref  PRIMARY        PRIMARY  dev.b.a_id  1

Answer 1

The explain output is pretty clear that it is using indexes at every stage. A full table scan would be indicated by “ALL” under the type column. This looks like it will use the indexes to access exactly those records from which you need data. (The counting is also done using only the index.) See here for more info on interpreting the output of EXPLAIN.