I encountered the same error on Windows 10 with Postgres freshly installed and its installation directory E:\PostgreSQL\bin and E:\PostgreSQL\include (which contains libpq-fe.h !) in the PATH variable.
I also tried the following (which helped in other cases) but the error remains:

Copy libpq-fe.h from E:\PostgreSQL\include to C:\Users\CVO\AppData\Local\Temp\v
copy libpq-fe.h near customer.v: idem
copy libpq-fe.h to C:\mingw-w64\bin\mingw64\x86_64-w64-mingw32\lib: idem
copy libpq.dll near customer.v and in local\temp\v : idem

It is clearly a PostgreSQL on Windows problem, doesn't happen on Linux.

So try installing libpq-dev or its equivalent for your OS:

For Ubuntu/Debian systems: sudo apt-get install libpq-dev
On Red Hat Linux (RHEL) systems: yum install postgresql-devel
For Mac Homebrew: brew install postgresql
For Mac MacPorts PostgreSQL: gem install pg -- --with-pg-config=/opt/local/lib/postgresql[version number]/bin/pg_config
For OpenSuse: zypper in postgresql-devel
For ArchLinux: pacman -S postgresql-libs

@walkingdevel Do you have a possibility to test this and eventually close this?

@ArtemkaKun, for now, I would keep it open. I will have a Windows device within a month, and then I will be able to check everything.

This also happens in macos with macports. I ran the required gem install pg -- --with-pg-config=/opt/local/lib/postgresql[version number]/bin/pg_config command and I can find libpq-fe.h manually if I

$ locate libpq-fe
/opt/local/include/postgresql13/libpq-fe.h

OK. So apparently, because https://github.com/vlang/v/blob/master/vlib/db/pg/pg.v will look at $pkgconfig('libpq') to find the correct flags or it will assume postgresql11 is used (macports uses /opt/local/):

$if $pkgconfig('libpq') {
	#pkgconfig --cflags --libs libpq
} $else {
	#flag -lpq
	#flag linux -I/usr/include/postgresql

	#flag darwin -I/opt/local/include/postgresql11
	#flag darwin -L/opt/local/lib/postgresql11

	#flag darwin -I/usr/local/opt/libpq/include
	#flag darwin -L/usr/local/opt/libpq/lib

	#flag darwin -I/opt/homebrew/include
	#flag darwin -L/opt/homebrew/lib

	#flag darwin -I/opt/homebrew/opt/libpq/include
	#flag darwin -L/opt/homebrew/opt/libpq/lib

	#flag windows -I @VEXEROOT/thirdparty/pg/include
	#flag windows -L @VEXEROOT/thirdparty/pg/win64
}

It's possible that because I'm using fish shell, macports didn't set PKG_CONFIG_PATH correctly. Once I ran

set -Ux PKG_CONFIG_PATH /opt/local/lib/postgresql13/pkgconfig/

the build error disappeared.

i get similar error when trying to run the example in orm doc

builder error: 'libpq-fe.h' not found

I am on win11 and dont have postgres locally (i am using a remote instance)

Even if your postgres server is remote, you still need to have the headers local for your build.

i managed to do that on wsl but not on windows.

If you have the files in wsl, you should be able to copy them to Windows. The pg driver source in V has these lines for Windows:

	#flag windows -I @VEXEROOT/thirdparty/pg/include
	#flag windows -L @VEXEROOT/thirdparty/pg/win64

So the .h files would go to your <v install dir>/thirdparty/pg/include directory (which you'll need to create). Not sure what goes in the win64 subdir, but likely a dll that's part of the Windows version of postgresql client.

We definitely need better docs on this...