Documentation Home
MySQL 5.7 Reference Manual
Related Documentation Download this Manual
PDF (US Ltr) - 35.1Mb
PDF (A4) - 35.2Mb
Man Pages (TGZ) - 256.4Kb
Man Pages (Zip) - 361.2Kb
Info (Gzip) - 3.4Mb
Info (Zip) - 3.4Mb
Excerpts from this Manual

8.2.1.1 WHERE Clause Optimization

This section discusses optimizations that can be made for processingWHERE clauses. The examples useSELECT statements, but the same optimizations apply forWHERE clauses inDELETE andUPDATE statements.

Note

Because work on the MySQL optimizer is ongoing, not all of the optimizations that MySQL performs are documented here.

You might be tempted to rewrite your queries to make arithmetic operations faster, while sacrificing readability. Because MySQL does similar optimizations automatically, you can often avoid this work, and leave the query in a more understandable and maintainable form. Some of the optimizations performed by MySQL follow:

  • Removal of unnecessary parentheses:

       ((a AND b) AND c OR (((a AND b) AND (c AND d))))-> (a AND b AND c) OR (a AND b AND c AND d)

  • Constant folding:

       (a<b AND b=c) AND a=5-> b>5 AND b=c AND a=5

  • Constant condition removal:

       (b>=5 AND b=5) OR (b=6 AND 5=5) OR (b=7 AND 5=6)-> b=5 OR b=6

  • Constant expressions used by indexes are evaluated only once.

  • COUNT(*) on a single table without aWHERE is retrieved directly from the table information forMyISAM andMEMORY tables. This is also done for anyNOT NULL expression when used with only one table.

  • Early detection of invalid constant expressions. MySQL quickly detects that someSELECT statements are impossible and returns no rows.

  • HAVING is merged withWHERE if you do not useGROUP BY or aggregate functions (COUNT(),MIN(), and so on).

  • For each table in a join, a simplerWHERE is constructed to get a fastWHERE evaluation for the table and also to skip rows as soon as possible.

  • All constant tables are read first before any other tables in the query. A constant table is any of the following:

    • An empty table or a table with one row.

    • A table that is used with aWHERE clause on aPRIMARY KEY or aUNIQUE index, where all index parts are compared to constant expressions and are defined asNOT NULL.

    All of the following tables are used as constant tables:

    SELECT * FROM t WHEREprimary_key=1;SELECT * FROM t1,t2  WHERE t1.primary_key=1 AND t2.primary_key=t1.id;

  • The best join combination for joining the tables is found by trying all possibilities. If all columns inORDER BY andGROUP BY clauses come from the same table, that table is preferred first when joining.

  • If there is anORDER BY clause and a differentGROUP BY clause, or if theORDER BY orGROUP BY contains columns from tables other than the first table in the join queue, a temporary table is created.

  • If you use theSQL_SMALL_RESULT modifier, MySQL uses an in-memory temporary table.

  • Each table index is queried, and the best index is used unless the optimizer believes that it is more efficient to use a table scan. At one time, a scan was used based on whether the best index spanned more than 30% of the table, but a fixed percentage no longer determines the choice between using an index or a scan. The optimizer now is more complex and bases its estimate on additional factors such as table size, number of rows, and I/O block size.

  • In some cases, MySQL can read rows from the index without even consulting the data file. If all columns used from the index are numeric, only the index tree is used to resolve the query.

  • Before each row is output, those that do not match theHAVING clause are skipped.

Some examples of queries that are very fast:

SELECT COUNT(*) FROMtbl_name;SELECT MIN(key_part1),MAX(key_part1) FROMtbl_name;SELECT MAX(key_part2) FROMtbl_name  WHEREkey_part1=constant;SELECT ... FROMtbl_name  ORDER BYkey_part1,key_part2,... LIMIT 10;SELECT ... FROMtbl_name  ORDER BYkey_part1 DESC,key_part2 DESC, ... LIMIT 10;

MySQL resolves the following queries using only the index tree, assuming that the indexed columns are numeric:

SELECTkey_part1,key_part2 FROMtbl_name WHEREkey_part1=val;SELECT COUNT(*) FROMtbl_name  WHEREkey_part1=val1 ANDkey_part2=val2;SELECT MAX(key_part2) FROMtbl_name GROUP BYkey_part1;

The following queries use indexing to retrieve the rows in sorted order without a separate sorting pass:

SELECT ... FROMtbl_name  ORDER BYkey_part1,key_part2,... ;SELECT ... FROMtbl_name  ORDER BYkey_part1 DESC,key_part2 DESC, ... ;