Answer / Munnakumar

MySQL optimizes the DISTINCT keyword by eliminating duplicate rows as early as possible in the execution of a SELECT statement. To do this, it creates an intermediate temporary table with unique values and then selects from this table.nn- MySQL 5.6 or later versions use hash-based methods for optimizing DISTINCT queries with a smaller number of distinct values.nn- If the number of distinct values is large, MySQL will switch to a sort-based method (using sort_buffer_size and max_sort_length system variables) that sorts all rows first, then eliminates duplicates.

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