content_image/714292.jpg
Conversely, to get rows where the numbers are the same, the test should find differences within the tolerance value: 

mysql> SELECT i, SUM(d1) AS a, SUM(d2) AS b FROM t1 -> GROUP BY i HAVING ABS(a - b) <= 0.0001; 

<img src='content_image/714292.jpg'>

5 rows in set (0.03 sec) 

Floating-point values are subject to platform or implementation dependencies. Suppose that you execute the following statements: 

CREATE TABLE t1(c1 FLOAT(53,0), c2 FLOAT(53,0)); INSERT INTO t1 VALUES(’1e+52’,’-1e+52’); SELECT * FROM t1; 

On some platforms, the SELECT  statement returns inf  and -inf . On others, it returns 0  and -0 . 

An implication of the preceding issues is that if you attempt to create a replica by dumping table contents with mysqldump  on the source and reloading the dump file into the replica, tables containing floating-point columns might differ between the two hosts. 

## B.3.5 Optimizer-Related Issues 

MySQL uses a cost-based optimizer to determine the best way to resolve a query. In many cases, MySQL can calculate the best possible query plan, but sometimes MySQL does not have enough information about the data at hand and has to make “educated”  guesses about the data. 

For the cases when MySQL does not do the "right" thing, tools that you have available to help MySQL are: 

•Use the EXPLAIN  statement to get information about how MySQL processes a query. To use it, just add the keyword EXPLAIN  to the front of your SELECT  statement: 

mysql> EXPLAIN SELECT * FROM t1, t2 WHERE t1.i = t2.i; 

EXPLAIN  is discussed in more detail in Section 13.8.2, “EXPLAIN Statement” . 

•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; 

USE INDEX  and IGNORE INDEX  may also be useful. See Section 8.9.4, “Index Hints” . 

•Global and table-level STRAIGHT_JOIN . See Section 13.2.10, “SELECT Statement” . 

•You can tune global or thread-specific system variables. For example, 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” . 

