Question

Usage of SQL_CALC_FOUND_ROWS

Asked by: speedpacket

Hi,

I was a bit afraid of signing up, but wow - the answers on this board are absolutely fantastic, so I signed up and hopefully someone will be able to answer my question as well...

I have written a datagrid which needs to display data from a query.  The input is a query of course, and for that query, the data is selected / paginated.

One of the things we need is total row count for the query.  This query however can take ANY form and in some cases unfortunately features a UNION or otherwise...

My method for getting the total rowcount is attached.

As you can see this is a nasty problem when we're having LOTS of records(where I run into now: 2M+ records) because with the inner select it is going to be stored in memory for the outer select.

I read about using SQL_CALC_FOUND_ROWS on here (see related solution), but I don't really see how I can use this within this function is the result of $this->MakeSQLQuery() can take virtually any form...  What if that query contains a union?

Any idea on how I might be able to work around my issue?

thanks a lot!
David.



function getRowCount($args) {
            if(!is_null($this->_md["meta"]["rows"]["totalcount"])) {
                return $this->_md["meta"]["rows"]["totalcount"];
            } else {           
                global $db;   
                $sql = "SELECT COUNT(*) as counter FROM (" . $this->MakeSQLQuery() . ") countTable";                               
                $rs = $db->Execute($sql);               
                return $rs->fields["counter"];
            }
        }

                                  
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Asked On
2009-11-06 at 04:58:08ID24877695
Tags

mysql

,

total rowcount

,

total row count

,

limit

,

large tables

Topic

MySQL Server

Participating Experts
1
Points
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Comments
5

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Answers

 

by: ChristoferDutzPosted on 2009-11-06 at 05:53:01ID: 25759044

It seems the "SQL_CALC_FOUND_ROWS" Option is added directly after the select keyword and is generally only valuable, when using the LIMIT Keyword to limit the amount of results.

in this case at first a limited query is executed:
"SELECT * FROM myTable LIMIT 100"
then adding the "SQL_CALC_FOUND_ROWS" Keyword:
"SELECT SQL_CALC_FOUND_ROWS * FROM myTable LIMIT 100"
Would result in the same result, but you would be able to issue a second query directly after the first:
"SELECT FOUND_ROWS();"
Which would return the amount of rows the query would have returned, if the uery wasn't limited.
Don't know if this is universal for all databases, but It seems to work with MySQL

So for your problem, you could add the Keyword to your query returned by MakeSQLQuery() and simply call
"SELECT FOUND_ROWS();" after that. But I doubt that it would perform much better. Perhaps limiting the query returned by MakeSQLQuery() by adding a LIMIT 1 at the end would make MySQL only collect statistics and not the Table data not needed for calculating the number of reults.

 

by: speedpacketPosted on 2009-11-06 at 05:58:50ID: 25759093

Hi Christopher,

Thanks for the swift feedback!
I read soe further myself, and it does look as if I just need to add the SQL_CALC_FOUND_ROWS to the very first SELECT in case of a UNION, so I'll give it a chance and replace SELECT by SELECT SQL_CALC_FOUND_ROWS in the returned statement, executing that and then executing the SELECT FOUND_ROWS(); statement.

I hope this will work.

thanks for the feedback though!

 

by: speedpacketPosted on 2009-11-06 at 06:14:05ID: 25759236

Hi,

I tried again, and this seems to be working fine...
Also for unions :)

thanks for the feedback though!

        function getRowCount($args) {
            if(!is_null($this->_md["meta"]["rows"]["totalcount"])) {
                return $this->_md["meta"]["rows"]["totalcount"];
            } else {           
                global $db;   
               
                $sql = preg_replace("/SELECT/", "SELECT SQL_CALC_FOUND_ROWS", $this->MakeSQLQuery(), 1); 
                $temp = $temprs = $db->SelectLimit($sql, 1, 0, false); 
                
                                
                $rs = $db->Execute("SELECT found_rows() as 'counter'");
                 return $rs->fields["counter"];
            }
        }
                                              
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by: ChristoferDutzPosted on 2009-11-11 at 01:27:35ID: 25793415

Thanks for no points buddy ;-)

 

by: speedpacketPosted on 2009-11-11 at 03:04:32ID: 25793962

I thought the idea was to assign points if you actually helped LOL
Although your comment was much appreciated, it does not relate to my question at all really...

What you wrote was what I knew as well - I was just looking for a solution to work with this in complicated queries using unions...

If you re-read this topic, I'm sure you'll see that your feedback doesn't really cover this question...  It's much appreciated indeed, but unless I don't understand this site (fair enough: it's my first time here!), I don't think your feedback should be considered a solution, and therefor I should not award points, or am I missing something?

20120131-EE-VQP-002

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