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full table scan for limit??
Posted by: h ch ()
Date: December 15, 2009 01:49AM

i am very puzzle about the question if the limit will let SQL optimizer to do full table scan.
and i read the following text from mysql-administation bible by someone,it says

Although the LIMIT clause can be useful, its implementation is very basic. In order to retrieve the information, mysqld processes a query as if there were no LIMIT, and stops when it reaches the row count it needs to.

that means when the query like " select * from mytable limit N" executing,it will look the data it want,but when after it reached the N'th record,it stop looking ,and just quit. that's means it did not do full table scan,

but i also readed another text about SQL_CALC_FOUND_ROWS usage,that's means it did do the full table scan,which one is right??can anybody help?

mysql> SELECT SQL_CALC_FOUND_ROWS rental_date, inventory_id,
-> customer_id, return_date FROM RENTAL LIMIT 1\G
| found_rows() |
| 16044 |

| count(*) |
| 16044 |

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Subject Views Written By Posted
full table scan for limit?? 4577 h ch 12/15/2009 01:49AM
Re: full table scan for limit?? 2158 Rick James 12/22/2009 12:04PM

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