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"ORDER BY" AND "LIMIT BY" CLAUSE PROBLEM: When a 'limit' clause is specified in a query along with group by and order by, optimizer chooses wrong index there by examining more number of rows than required. However without the 'limit' clause, optimizer chooses the right index. ANALYSIS: With respect to the query specified, range optimizer chooses the first index as there is a range present ( on 'a'). Optimizer then checks for an index which would give records in sorted order for the 'group by' clause. While checking chooses the second index (on 'c,b,a') based on the 'limit' specified and the selectivity of 'quick_condition_rows' (number of rows present in the range) in 'test_if_skip_sort_order' function. But, it fails to consider that an order by clause on a different column will result in scanning the entire index and hence the estimated number of rows calculated above are wrong (which results in choosing the second index). FIX: Do not enforce the 'limit' clause in the call to 'test_if_skip_sort_order' if we are creating a temporary table. Creation of temporary table indicates that there would be more post-processing and hence will need all the rows. This fix is backported from 5.6. This problem is fixed in 5.6 as part of changes for work log #5558 mysql-test/r/subselect.result: Changes for Bug#11762052 results in the correct number of rows. sql/sql_select.cc: Do not pass the actual 'limit' value if 'need_tmp' is true.
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