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## EXPLAIN Output Columns 

•EXPLAIN Extra Information 

•EXPLAIN Output Interpretation 

This section describes the output columns produced by EXPLAIN . Later sections provide additional information about the type  and Extra  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. 

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• id  (JSON name: select_id ) 

## Note 

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 . 

• select_type  (JSON name: none) 

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. 

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