1771 EXPLAIN Output Format This value occurs with  EXPLAIN FOR CONNECTION  when the optimizer has not finished creating the execution plan for the statement executing in the named connection. If execution plan output comprises multiple lines, any or all of them could have this  Extra  value, depending on the progress of the optimizer in determining the full execution plan. MySQL found no good index to use, but found that some of indexes might be used after column values from preceding tables are known. For each row combination in the preceding tables, MySQL checks whether it is possible to use a  range  or  index_merge  access method to retrieve rows. This is not very fast, but is faster than performing a join with no index at all. The applicability criteria are as described in Section 8.2.1.2, “Range Optimization” , and Section 8.2.1.3, “Index Merge Optimization” , with the exception that all column values for the preceding table are known and considered to be constants. Indexes are numbered beginning with 1, in the same order as shown by  SHOW INDEX  for the table. The index map value  N  is a bitmask value that indicates which indexes are candidates. For example, a value of  0x19  (binary 11001) means that indexes 1, 4, and 5 are considered. This indicates that the row applies to the recursive  SELECT  part of a recursive common table expression. See Section 13.2.15, “WITH (Common Table Expressions)” . Rematerialize (X,...)  is displayed in the  EXPLAIN  row for table  T , where  X  is any lateral derived table whose rematerialization is triggered when a new row of  T  is read. For example: The content of the derived table is rematerialized to bring it up to date each time a new row of  t  is processed by the top query. This indicates how many directory scans the server performs when processing a query for INFORMATION_SCHEMA  tables, as described in Section 8.2.3, “Optimizing INFORMATION_SCHEMA Queries” . The value of  N  can be 0, 1, or  all . • Scanned  N  databases  (JSON property:  message ) • Rematerialize  (JSON property:  rematerialize ) • Recursive  (JSON property:  recursive ) • Range checked for each record (index map:  N )  (JSON property:  message ) SELECT ... FROM t, LATERAL ( derived table that refers to t ) AS dt ...