How to Avoid Full Table Scans

The output from EXPLAIN shows ALL in the type column when MariaDB uses a full table scan to resolve a query. 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.
• You are comparing indexed columns with constant values and MariaDB has calculated (based on the index tree) that the constants cover too large a part of the table and that a table scan would be faster. See , "How MariaDB Optimizes WHERE Clauses".
• You are using a key with low cardinality (many rows match the key value) through another column. In this case, MariaDB assumes that by using the key it probably will do many key lookups and that a table scan would be faster.

For small tables, a table scan often is appropriate and the performance impact is negligible. 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 , "ANALYZE TABLE Syntax".
• Use FORCE INDEX for the scanned table to tell MariaDB that table scans are very expensive compared to using the given index:

  SELECT * FROM t1, t2 FORCE INDEX (index_for_column)
   WHERE t1.col_name=t2.col_name;
  

  See , "Index Hint Syntax".

• Start mysqld with the --max-seeks-for-key=1000 option or use SET max_seeks_for_key=1000 to tell the optimizer to assume that no key scan causes more than 1,000 key seeks. See , "Server System Variables".

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