What is the most efficient way to estimate the number of returned rows in SQL without a "Select count(*)" ?

Question

I want to do something like on Google/MSN/Yahoo where they display in advance the number of estimated rows/pages without actually counting them.

Answer 1

I found this is the mysql docs:

A SELECT statement may include a LIMIT clause to restrict the number of rows the server returns to the client. In some cases, it is desirable to know how many rows the statement would have returned without the LIMIT, but without running the statement again. To get this row count, include a SQL_CALC_FOUND_ROWS option in the SELECT statement, then invoke FOUND_ROWS() afterward:

mysql> SELECT SQL_CALC_FOUND_ROWS * FROM tbl_name
WHERE id > 100 LIMIT 10;

mysql> SELECT FOUND_ROWS();

So you can get the first n items and also the total number of items from just 2 lines. I'm guessing SQL_CALC_FOUND_ROWS and FOUND_ROWS() (or some equivalent) is available in mostSQL databases.

Answer 2

SQL does not know how to estimate a value. You would need to supply your own equation to estimate the number of rows. Otherwise just let SQL count the exact number.

Answer 3

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