Expressions: LIKE operator is slow

[wonder-sk]

QGIS rendering may get quite slow with the use of expressions using LIKE.

Internally expressions use QRegExp when evaluating LIKE - this class has now a replacement in Qt5 - QRegularExpression. The new class has better API (easier to cache the engine for the regexp) and also a faster engine (based on PCRE library instead of custom one implemented within Qt). It looks like we need to switch to the new class + add caching of the regexp instance where possible (when the right side is a literal string).

Some platforms seem to be more prone to the slowness - e.g. iOS - see MerginMaps/mobile#433

[roya0045]

Is this on anyone's radar? I can try to cook up something.

[nyalldawson]

This is fixed now -- everything in core is migrated to QRegularExpression