BUG Update DataQuery::exists to return false when limit causes no result to be returned

[maxime-rainville]

This was raised by @mfendeksilverstripe.

While upgrading from 4.5 to 4.7, he discovered that DataList who are constrained by a limit into retuning 0 results, would still be considered "truthy" when a if statement was applied to them in an SS template:

4.5
<% if $Results %> <-- DataList which yields 0 results is treated as false
4.7
<% if $Results %> <-- DataList which yields 0 results is treated as true

[maxime-rainville]

I've updated the scenarios here to test both the DataList exist and the DataQuery exists. I've added scenarios for the limit and added explicit message for each assertion.

[bergice]

LGTM.

Needs a lint fix.

Wondering if it should also handle cases where ->limit(0) is applied? In that case this won't work because of vendor/silverstripe/framework/src/ORM/Connect/MySQLQueryBuilder.php:33.

[ScopeyNZ]

Why would a limit return zero results? Is this in the instance where you're fetching page 3 or something, and your limit is LIMIT 20 OFFSET 40, but you only have 35 results?

I wonder how the engine handles an exists statement that has a limit - would you lose potential performance benefits when indexes may not be consulted?

I guess it'll have to be extra important that parts of the code that are looking for any record regardless of pagination ensure they have not set a limit on their DataQuery.

[maxime-rainville]

Is this in the instance where you're fetching page 3 or something, and your limit is LIMIT 20 OFFSET 40, but you only have 35 results?

Yes. We can have a look at the performance impact, but this is a regression and we need to find some fix.

You can't trigger this behaviour with setLimit(0) because that removes any limit.

[emteknetnz]

Looks good

Weird behaviour is probably linked to this change 9159137

It's consistent with the original report (on slack) about count() working correct, though exists() being strange

The original report also had an replication example provided of $results->limit(4, 12811); which is a small limit with a huge offset, which is similar to what Guy mentioned earlier re LIMIT 20 OFFSET 40 on a 35 row table