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In some cases, at the time that we're doing an NFA-based precheck of whether a backref subexpression can match at a particular place in the string, we already know which substring the referenced subexpression matched. If so, we might as well forget about the NFA and just compare the substring; this is faster and it gives an exact rather than approximate answer. In general, this optimization can help while we are prechecking within the second child expression of a concat node, while the capture was within the first child expression; then the substring was saved during cdissect() of the first child and will be available to NFA checks done while cdissect() recurses into the second child. It can help quite a lot if the tree looks like concat / \ capture concat / \ expensive stuff backref as we will be able to avoid recursively dissecting the "expensive stuff" before discovering that the backref isn't satisfied with a particular midpoint that the lower concat node is testing. This doesn't help if the concat tree is left-deep, as the capture node won't get set soon enough (and it's hard to fix that without changing the engine's match behavior). Fortunately, right-deep concat trees are the common case. Patch by me, reviewed by Joel Jacobson Discussion: https://postgr.es/m/661609.1614560029@sss.pgh.pa.us |
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