Hello.
Excellent library. Thank you.

I tried to build tests through GNU make.
I've got a lot compile time errors.

g++ -g -D_DEBUG -O2 -I/usr/include -I/usr/local/include -I/usr/include/postgresql -I/usr/include/postgresql/12/server -I../include -std=c++11 -x c++-header -o stdafx.h.gch stdafx.h
g++ -c -g -D_DEBUG -O2 -I/usr/include -I/usr/local/include -I/usr/include/postgresql -I/usr/include/postgresql/12/server -I../include -std=c++11 -o TestPostgres.o TestPostgres.cpp
In file included from /usr/include/postgresql/12/server/nodes/parsenodes.h:27,
from /usr/include/postgresql/12/server/utils/acl.h:36,
from /usr/include/postgresql/12/server/catalog/objectaddress.h:18,
from /usr/include/postgresql/12/server/catalog/pg_type.h:24,
from ../include/qtl_postgres.hpp:31,
from TestPostgres.cpp:8:
/usr/include/postgresql/12/server/nodes/primnodes.h:200:2: error: ‘Datum’ does not name a type; did you mean ‘outDatum’?
200 | Datum constvalue; /* the constant's value */
| ^~~~~
| outDatum
In file included from /usr/include/postgresql/12/server/utils/acl.h:37,
from /usr/include/postgresql/12/server/catalog/objectaddress.h:18,
from /usr/include/postgresql/12/server/catalog/pg_type.h:24,
from ../include/qtl_postgres.hpp:31,
from TestPostgres.cpp:8:
/usr/include/postgresql/12/server/parser/parse_node.h:263:2: error: ‘ErrorContextCallback’ does not name a type
263 | ErrorContextCallback errcallback;
| ^~~~~~~~~~~~~~~~~~~~
....

Software versions:

Ubuntu 20.04
GNU Make 4.2.1
g++ (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0
postgresql-client-12 (12.12-0ubuntu0.20.04.1)
postgresql-client-common (214ubuntu0.1)
postgresql-server-dev-12:amd64 (12.12-0ubuntu0.20.04.1)
libpgtypes3:amd64 (12.12-0ubuntu0.20.04.1)
libecpg6:amd64 (12.12-0ubuntu0.20.04.1)
libecpg-compat3:amd64 (12.12-0ubuntu0.20.04.1)
libecpg-dev (12.12-0ubuntu0.20.04.1) …

Seems missing #include in <qtl/qtl_postgres.hpp>

//...
extern "C"
{
#include <postgres.h> // NEED THIS FOR COMPILE //
#include <c.h>
#include <catalog/pg_type.h>
}

//...

Building was finished OK after this addition.