Load libpq.dll after just checking if it exists. Do not check what LoadLibrary returns. This is quite risky, but there seem to be cases where LoadLibrary returns 0 although libpq.dll was loaded. See http://www.heidisql.com/forum.php?t=21039

Author: ansgarbecker

source/dbconnection.pas

@@ -2097,11 +2097,11 @@ procedure TPgConnection.DoBeforeConnect;
   // Each connection has its own library handle
   if LibPqHandle = 0 then begin
     LibWithPath := ExtractFileDir(Application.ExeName) + '\' + LibPqPath;
-    Log(lcDebug, f_('Loading library file %s ...', [LibWithPath]));
-    LibPqHandle := LoadLibrary(PWideChar(LibWithPath));
-    if LibPqHandle = 0 then
-      raise EDatabaseError.CreateFmt(_('Cannot find a usable %s. Please launch %s from the directory where you have installed it.%s'), [LibPqPath, ExtractFileName(ParamStr(0)), CRLF+CRLF+SysErrorMessage(System.GetLastError)])
+    if not FileExists(LibWithPath) then
+      raise EDatabaseError.CreateFmt(_('Cannot find a usable %s. Please launch %s from the directory where you have installed it.'), [LibPqPath, ExtractFileName(ParamStr(0))])
     else begin
+      Log(lcDebug, f_('Loading library file %s ...', [LibWithPath]));
+      LibPqHandle := LoadLibrary(PWideChar(LibWithPath));
       AssignProc(@PQconnectdb, 'PQconnectdb');
       AssignProc(@PQerrorMessage, 'PQerrorMessage');
       AssignProc(@PQresultErrorMessage, 'PQresultErrorMessage');