Any time you wonder about slow performance in an iOS (or Mac) app, you should
reach for Instruments and run the CPU Profiler.
>From looking at your code, I don’t see anything obvious. The map function
>should be fast. NSPredicates aren’t terribly fast, but it shouldn’t take 5ms
>to run that predicate. I’m curious to hear where Instruments says the
>bottlenecks are.
However, that’s a poorly optimized query: it’s going to do a linear scan of the
index every time it runs, instead of taking advantage of b-tree search. Your
search criterion is the (fromId, toId) pair, so that’s what you should be
indexing on. What I would do is
emit( [min(doc.fromId, doc.toID), max(doc.fromID, doc.toId)],
doc.messageDate )
Then query with key = [min(fromId, toID), max(fromID, toId)], and sort the
results by row.value (which will be the date.)
In case it’s not clear: the reason for the min and max calls is to create the
same key whichever order the fromId/toId appear in, since the query code you
gave doesn’t care about the ordering.
—Jens
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