Poorly-thought-out handling of double variables in pgbench

While testing 7a622b273 I happened to notice this:

\set x greatest(3, 2, 4.9)
create table mytab (x numeric);
insert into mytab values(:x);

results in this table:

x
----------------------
4.900000000000000355
(1 row)

The reason for that is that the result of a "double" calculation
is coerced to text like this:

sprintf(res, "%.18e", result.u.dval);

apparently on the theory that this will result in being able to convert it
back to double with no loss of low-order bits. But of course the last two
or three digits of such output are pretty much garbage to the naked eye.
Then what gets interpolated as the variable value is something like
'4.900000000000000355e+00'.

I think this is a very poor decision; it's going to cause surprises and
probably bug reports. Moreover, if we were testing this behavior in the
buildfarm (which we are not, but only because the test coverage for
pgbench is so shoddy), we would be seeing failures, because those
low-order digits are going to be platform dependent.

The only value of doing it like this would be if people chained multiple
floating-point calculations in successive pgbench \set commands and needed
full precision to be retained from one \set to the next ... but how likely
is that?

A minimum-change fix would be to print only DBL_DIG digits here. A better
answer, perhaps, would be to store double-valued variables in double
format to begin with, coercing to text only when and if the value is
interpolated into a string. That's probably a bigger change than we
want to be putting in right now, though I'm a bit tempted to go try it.

Thoughts?

BTW, just to add insult to injury, the debug() function prints double
values with "%.f", which evidently had even less thought put into it.
That one should definitely be %g with DBL_DIG precision.

regards, tom lane