I think this is a good feature that would be useful to JDBC and more.
I don't know the surrounding code very well, but the patch looks good to me.
I agree with Tom Lane that the name of the variable is too verbose.
Maybe "auto_binary_types" is enough. Do we gain much by prefixing
"result_format_"? Wouldn't we use the same variable, if we support
binary inputs one day?
It is nice that the patch comes with the test module. The name
"libpq_extended" sounds a bit vague to me. Maybe it's a better idea
to call it "libpq_result_format" and test "format=1" in it as well.
My last nitpicking about the names is the "test-result-format"
command. All the rest of the test modules name the commands with
underscores. It would be nicer if this one complies.
There is one place that needs to be updated on the Makefile of the test:
> +subdir = src/test/modules/libpq_pipeline
s/pipeline/extended/
Then the test runs successfully.