We recently noticed that when we got to about 15,000 posts in our blog that it was not possible to open the admin->edit section on the largest blog. Some investigation led to the conclusion that the procedure function calc_max() located in _class_itemlist.php was excuting such a large query that our mysql system was balking at handling the request.
The query being executed was :

$this->request = "SELECT  ID, post_author, post_issue_date,	 post_mod_date, post_status, post_locale, post_content, post_title, post_urltitle, post_url, post_category, post_autobr, post_flags, post_wordcount, post_comments, post_renderers, post_karma
			FROM ($tableposts INNER JOIN $tablepostcats ON ID = postcat_post_ID)
			INNER JOIN $tablecategories ON postcat_cat_ID = cat_ID ";

What the procedure calc_max was doing was to count the num_rows to see how many entries there were in this particular blog.

Since all that was being determined was the count, we recommend the follwoing changes in _class_itemlist.php:

At about line 486:

$this->request = "SELECT  ID, post_author, post_issue_date,	 post_mod_date, post_status, post_locale, post_content, post_title, post_urltitle, post_url, post_category, post_autobr, post_flags, post_wordcount, post_comments, post_renderers, post_karma
		FROM ($tableposts INNER JOIN $tablepostcats ON ID = postcat_post_ID)						INNER JOIN $tablecategories ON postcat_cat_ID = cat_ID ";						
$this->request_alt = "SELECT  count(*) as total_num_posts
		FROM ($tableposts INNER JOIN $tablepostcats ON ID = postcat_post_ID)
		INNER JOIN $tablecategories ON postcat_cat_ID = cat_ID ";
	if( $blog == 1 )
		{ // Special case: we aggregate all cats from all blogs
		$this->request .= 'WHERE 1 ';
		$this->request_alt  .= 'WHERE 1 ';
		}
		else
		{
		$this->request .= 'WHERE cat_blog_ID = '. $blog;
		$this->request_alt  .= 'WHERE cat_blog_ID = '. $blog;
		}
		$this->request .= $where. " ORDER BY post_$orderby $limits  ";
		$this->request_alt  .= $where;

{to form a new request that asks for the count(*) rather than all the data.

and additional changes to calc_max as follows}

	function calc_max()
	{
		global $DB;
//print"IIIIIIIIIIIIIIIIIIII<BR>";
		if( $this->preview )
			return 1;	// 1 row in preview mode
// This is a ridiculous bit of code. It selects every post in the blog in great detail, just to calculate the number.
// A count request would have been a lot simpler
//For now I have just set this to return 100, more thqan enough for my purposes  JWN 8/11/05
		$nxt_request = $this->request_alt;
	//echo "IIIIIIII".$nxt_request."IIIIIIIII<BR>";		
//		if( $pos = strpos(strtoupper($this->request), 'LIMIT'))
//		{ // Remove the limit form the request
//			$nxt_request = substr($this->request, 0, $pos);
//		}
//		echo "IIIIIIII".$nxt_request."IIIIIIIII<BR>";

		$DB->query( $nxt_request );
		$return_array_alt = $DB->get_col( $query = NULL, $x=0, $title = '' );
		//print_r($return_array_alt);
		//$this->total_num_posts = $DB->num_rows;
		$this->total_num_posts = $return_array_alt[0];
//print "total_num_posts = $total_num_posts:$DB->total_num_posts:$this->total_num_posts";
		$this->max_paged = intval( ($this->total_num_posts-1) / max($this->posts_per_page, $this->result_num_rows)) +1;
		if( $this->max_paged < 1 )
			$this->max_paged =1;
	}

Hope you can use this for new version.