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How to avoid a full table scan in MySQL?
For large tables, try the following techniques to avoid having the optimizer incorrectly choose a table scan: Use ANALYZE TABLE tbl_name to update the key distributions for the scanned table. See Section 13.7.3.1, “ANALYZE TABLE Statement” .
Why are there no restrictions on indexed columns in MySQL?
This usually happens under the following conditions: The table is so small that it is faster to perform a table scan than to bother with a key lookup. This is common for tables with fewer than 10 rows and a short row length. There are no usable restrictions in the ON or WHERE clause for indexed columns.
How to remove nonkeys from MySQL range scan?
The extraction process for key key1 is as follows: Start with original WHERE clause: Remove nonkey = 4 and key1 LIKE ‘%b’ because they cannot be used for a range scan. The correct way to remove them is to replace them with TRUE, so that we do not miss any matching rows when doing the range scan.
What happens in the extraction process in MySQL?
During the extraction process, conditions that cannot be used for constructing the range condition are dropped, conditions that produce overlapping ranges are combined, and conditions that produce empty ranges are removed. Consider the following statement, where key1 is an indexed column and nonkey is not indexed:
What are the limits of MySQL internal temporary tables?
Internal Temporary Table Use in MySQL Limits on Number of Databases and Tables Limits on Table Size Limits on Table Column Count and Row Size Optimizing for InnoDB Tables Optimizing Storage Layout for InnoDB Tables
How to optimize SELECT statements in MySQL?
Optimizing SQL Statements Optimizing SELECT Statements WHERE Clause Optimization Range Optimization Index Merge Optimization Hash Join Optimization Engine Condition Pushdown Optimization Index Condition Pushdown Optimization Nested-Loop Join Algorithms Nested Join Optimization Outer Join Optimization Outer Join Simplification