Small catcache optimization

Hi,

Heikki's catcache rehashing stuff reminded me that I'd posted an
optimization to catcache (20121220153555(dot)GH4303(at)awork2(dot)anarazel(dot)de) some
time back which I didn't have energy to pursue at that point.

I've brushed the patch up a bit and verified it still gives be a
performance improvement. It's still about 2% in a readonly pgbench on my
elderly laptop.

There's basically two tricks in the patch:
1) Don't always copy the cache's ScanKey to the stack. Instead pass
an array of arguments around. That get's rid of a good amount of
memcpy()ing in the common, cached case.
2) If we have to memcpy() because we need to pass a ScanKey to
systable_*, copy only cache->cc_nkey * sizeof(ScanKeyData) instead of
always copying the maximum size.

I'd be nicer to get rid of the mostly copied HeapKeyTestArg, but I don't
immediately see how.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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