Hardware and performance

Dear all:

It looks like by default, Moodle 1.9.x would set up a cron job that would run a query every 15 minutes to clean out the "grades history" table. According to our system administrators, this doesn't appear to cause any site slow down or a serious bump in CPU or overall activity. But it seems that it is not a really efficient query (especially when the grade history table gets bigger). Here is a brief sample report:

===========================

User@Host: xxxxx @ localhost [127.0.0.1]
Query_time: 2.411335 Lock_time: 0.000107 Rows_sent: 0 Rows_examined: 0
SET timestamp=1257566407;
DELETE FROM mdl_grade_grades_history WHERE time modified < 1226030404;

User@Host: xxxxx @ localhost [127.0.0.1]
Query_time: 2.314545 Lock_time: 0.000041 Rows_sent: 0 Rows_examined: 0
SET timestamp=1257567359;
DELETE FROM mdl_grade_grades_history WHERE timemodified < 1226031356;

mysql> explain select * FROM mdl_grade_grades_history WHERE timemodified < 1226031356;
--- [snip]

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A possible solution would be to add an index on time modified:

ALTER TABLE `mdl_grade_grades_history` ADD INDEX `timemodified` (`timemodified`) ;

My questions are:

1. Is it really necessary to run this query every 15 minutes?

2. If that's the case, would it be a good idea to maybe modify the installation script so that it would create a index on timemodified with mdl_grade_grades_history by default?

3. Any other thoughts or comments about this?

Thanks!