Why the rows returns by “explain” is not equal to count()?

Question

    mysql> select count(*) from table where relation_title=\'xxxxxxxxx\';
+----------+
| count(*) |
+----------+
|  1291958 |
+----------+

mysql> explain sele         


        

Answer 1

Execute ANALYZE TABLE table_name; - it will update statistics that EXPLAIN uses, and you'll get correct numbers. For example: when there is no data in table at all, EXPLAIN will suggest that this table is empty and optimize queries to filter first based on that table (as it doesn't read anything from disk, memory and so on). Then when data will be loaded if you don't execute ANALYZE TABLE table_name;, optimizer still suggests that table is still empty, and does not use an optimal execution plan for query. EXPLAIN behaves same way - it doesn't look for current count of rows in table, it looks for statistics generated by ANALYZE TABLE table name (that is executed automatically in some situations - 1/16 of count of rows in table changed for example).