I did some benchmarking earlier in the week for the new patch which was just commited to allow formatting in the log_line_prefix string. In version 0.4 of the patch there was some performance regression as I was doing appendStringInfo(buf, "%*s", padding, variable); instead of appendStringInfoString(buf, variable); This regression was fixed in a later version of the patch by only using appendStringInfo when the padding was 0.
Today I started looking through the entire source tree to look for places where appendStringInfo() could be replaced by appendStringInfoString(), I now have a patch which is around 100 KB in size which changes a large number of these instances.
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Also on making the changes I noticed a possible small bug in the code that could cause a crash if for some reason a translation contained a %s. I know it is an unlikely scenario, never-the-less here is a patch to fix it.