I benchmarked this, using your testbed and comparing to libc sprintf (Ubuntu GLIBC 2.27-0ubuntu3) and another implementation I know [1], all compiled with gcc 5.4.0 with -O2. I used bigger decimals in one of the formats, but otherwise they are the same as yours. Here is the table of conversion time relative to libc:
format pg stb
("%2$.*3$f %1$d\n", 42, 123.456, 2) 1.03 -
("%.*g", 15, 123.456) 1.08 0.31
("%10d", 15) 0.63 0.52
("%s", "012345678900123456789001234 2.06 6.20
("%d 012345678900123456789001234567 2.03 1.81
("%1$d 0123456789001234567890012345 1.34 -
("%d %d", 845879348, 994502893) 1.97 0.59
Surprisingly, our implementation is twice faster than libc on "%10d". Stb is faster than we are with floats, but it uses its own algorithm for that. It is also faster with decimals, probably because it uses a two-digit lookup table, not one-digit like we do. Unfortunately it doesn't support dollars.
1. https://github.com/nothings/stb/blob/master/stb_sprintf.h
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Alexander Kuzmenkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company
