避免 全表扫描 full table scan

 1、ON  WHERE 有索引字段

MySQL :: MySQL 8.0 Reference Manual :: MySQL Glossary https://dev.mysql.com/doc/refman/8.0/en/glossary.html#glos_full_table_scan

full table scan

An operation that requires reading the entire contents of a table, rather than just selected portions using an index. Typically performed either with small lookup tables, or in data warehousing situations with large tables where all available data is aggregated and analyzed. How frequently these operations occur, and the sizes of the tables relative to available memory, have implications for the algorithms used in query optimization and managing the buffer pool.

The purpose of indexes is to allow lookups for specific values or ranges of values within a large table, thus avoiding full table scans when practical.

See Also buffer pool, index.

MySQL :: MySQL 8.0 Reference Manual :: 8.2.1.23 Avoiding Full Table Scans https://dev.mysql.com/doc/refman/8.0/en/table-scan-avoidance.html

8.2.1.23 Avoiding Full Table Scans

The output from EXPLAIN shows ALL in the type column when MySQL 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 MySQL 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 Section 8.2.1.1, “WHERE Clause Optimization”.

• You are using a key with low cardinality (many rows match the key value) through another column. In this case, MySQL assumes that by using the key probably requires 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 Section 13.7.3.1, “ANALYZE TABLE Statement”.

• Use FORCE INDEX for the scanned table to tell MySQL 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 Section 8.9.4, “Index Hints”.

• 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 Section 5.1.8, “Server System Variables”.