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The MySQL Query Optimizer When you issue a query that selects rows, MySQL analyzes it to see if anyoptimizations can be used to process the query more quickly. In this section,we'll look at how the query optimizer works. For additional informationabout optimization measures that MySQL takes, consult the optimization chapterin the MySQL Reference Manual. The MySQL query optimizer takes advantage of indexes, of course, but it alsouses other information.

For example, if you issue the following query, MySQLwill execute it very quickly, no matter how large the table is: SELECT * FROM tbl_name WHERE 0; In this case, MySQL looks at the WHERE clause, realizes that no rowscan possibly satisfy the query, and doesn't even bother to search thetable. You can see this by issuing an EXPLAIN statement, which tellsMySQL to display some information about how it would execute a SELECTquery without actually executing it. To use EXPLAIN, justput the word EXPLAIN in front of the SELECT statement: mysql>EXPLAIN SELECT * FROM tbl_name WHERE 0 G*************************** 1. Download Game Online Point Blank Pc Download. Row *************************** id: 1 select_type: SIMPLE table: NULL type: NULLpossible_keys: NULL key: NULL key_len: NULL ref: NULL rows: NULL Extra: Impossible WHERE Normally, EXPLAIN returns more information than that, including non- NULL information about the indexes that will be used to scan tables, the types of joins that will be used, and estimates of the number of rows that will need to be examined from each table. How the Optimizer Works The MySQL query optimizer has several goals, but its primary aims are to useindexes whenever possible and to use the most restrictive index in order toeliminate as many rows as possible as soon as possible. That last part mightsound backward and nonintuitive.

After all, your goal in issuing a SELECT statement is to find rows, not to reject them. The reason theoptimizer tries to reject rows is that the faster it can eliminate rows fromconsideration, the more quickly the rows that do match your criteria can befound. Queries can be processed more quickly if the most restrictive tests canbe done first. Suppose that you have a query that tests two columns, each ofwhich has an index on it: SELECT col3 FROM mytableWHERE col1 = 'some value' AND col2 = 'some other value'; Suppose also that the test on col1 matches 900 rows, the test on col2 matches 300 rows, and that both tests together succeed on 30 rows. Fha 3510 Drivers. Testing col1 first results in 900 rows that must be examined to findthe 30 that also match the col2 value.

That's 870 failed tests.Testing col2 first results in 300 rows that must be examined to findthe 30 that also match the col1 value. That's only 270 failedtests, so less computation and disk I/O is required. As a result, the optimizerwill test col2 first because doing so results in less work overall. You can help the optimizer take advantage of indexes by using the followingguidelines: Try to compare columns that have the same data type. Whenyou use indexed columns in comparisons, use columns that are of the same type.Identical data types will give you better performance than dissimilar types.

Forexample, INT is different from BIGINT. CHAR(10) isconsidered the same as CHAR(10) or VARCHAR(10) but differentfrom CHAR(12) or VARCHAR(12). If the columns you'recomparing have different types, you can use ALTER TABLE tomodify one of them so that the types match. Try to make indexed columns stand alone in comparisonexpressions.

If you use a column in a function call or as part of amore complex term in an arithmetic expression, MySQL can't use the indexbecause it must compute the value of the expression for every row. Sometimesthis is unavoidable, but many times you can rewrite a query to get the indexedcolumn to appear by itself. The following WHERE clauses illustrate how this works. They areequivalent arithmetically, but quite different for optimization purposes: WHERE mycol = 'Mac' AND last_name ALTER TABLE member ADD INDEX (expiration); Then try EXPLAIN with each form of the expression to see what kindof execution plans the optimizer comes up with: mysql>EXPLAIN SELECT * FROM MEMBER ->WHERE TO_DAYS(expiration) - TO_DAYS(CURDATE()) EXPLAIN SELECT * FROM MEMBER ->WHERE TO_DAYS(expiration) EXPLAIN SELECT * FROM MEMBER ->WHERE expiration SELECT t1.i1, t2.i2 FROM t1, t2 WHERE t1.i1 = t2.i2;+------+------+ i1 i2 +------+------+ 1 1 2 2 3 3 4 4 5 5 . With no indexes on either of the tables, EXPLAIN produces this result: mysql>EXPLAIN SELECT t1.i1, t2.i2 FROM t1, t2 WHERE t1.i1 = t2.i2 G*************************** 1.