EXPLAIN Output Format EXPLAIN Output Columns Each output row from  EXPLAIN  provides information about one table. Each row contains the values summarized in Table 8.1, “EXPLAIN Output Columns” , and described in more detail following the table. Column names are shown in the table’s first column; the second column provides the equivalent property name shown in the output when  FORMAT=JSON  is used. This section describes the output columns produced by  EXPLAIN . Later sections provide additional information about the  type  and  Extra  columns. •EXPLAIN Output Interpretation •EXPLAIN Extra Information Column JSON Name Meaning id select_id The  SELECT  identifier select_type None The  SELECT  type table table_name The table for the output row partitions partitions The matching partitions type access_type The join type possible_keys possible_keys The possible indexes to choose key key The index actually chosen key_len key_length The length of the chosen key ref ref The columns compared to the index rows rows Estimate of rows to be examined filtered filtered Percentage of rows filtered by table condition Extra None Additional information Table 8.1 EXPLAIN Output Columns Note • id  (JSON name:  select_id ) • select_type  (JSON name: none) JSON properties which are  NULL  are not displayed in JSON-formatted  EXPLAIN output. The  SELECT  identifier. This is the sequential number of the  SELECT  within the query. The value can be NULL  if the row refers to the union result of other rows. In this case, the  table  column shows a value like  <union M , N >  to indicate that the row refers to the union of the rows with  id  values of  M  and  N . The type of  SELECT , which can be any of those shown in the following table. A JSON-formatted EXPLAIN  exposes the  SELECT  type as a property of a  query_block , unless it is  SIMPLE  or  PRIMARY . The JSON names (where applicable) are also shown in the table. select_type  Value JSON Name Meaning SIMPLE None Simple  SELECT  (not using  UNION or subqueries) 1763