$DB->get_records uses a lot of Peak RAM (with Postgres)

This came from investigating group/overview.php which was running out of memory on one of our huge courses.

Eventually I boiled it down to this test script https://gist.github.com/timhunt/e5fa1174e920ff266db07e8fcfd4cef8 (also attached). You can save that in the root of your Moodle and run it. You need to have Admin -> Development -> Debugging -> Performance info turned on, and then compare the 'RAM' and 'RAM peak' reported for different $methods.

The results I get (64-bit Windows 10, Postgres 11) are:

method              RAM (MB)      RAM peak (MB)

none                      39            40.2

array_fill_object        321           322.1

get_records              369           554

get_recordset            352.8         354

(array_fill_object_other 449           450.1)

The first thing to note is that loading the arrays from the DB causes some overhead compared to just building the arrays in PHP memory.

However, the real problem is the huge 'RAM peak' for get_records. This can cause out-of-memory errors on scripts that reall ought to be able to run in the available RAM. It also means that using get_recordset then immediately copying the data to an array is more effecient that get_records, whereas to me, those two thigns should be logically equivalent.

So, if anyone can fix this, it might be quite a quick perforamance win. (However, I did not look at running time of the scripts. I was just focussed on memory use.)

(The other thing to note is, PHP is weird. Generating the array in PHP using sprintf is horribly inefficient for reasons I don't understand. That must be some sort of PHP bug!)