db.pg: fix using postgresql on windows, improve instructions for installing it (#20053)

vlib/db/pg/README.md

@@ -7,7 +7,7 @@ Before you can use this module, you must first have PostgreSQL installed on your
 To do this, find your OS and perform the actions listed.
 
 > **Note**
-> These instructions are meant only as a convenience. If your OS is not listed 
+> These instructions are meant only as a convenience. If your OS is not listed
 > or you need extra help, [go here](https://www.postgresql.org/download/).
 
 ### Fedora 31
@@ -45,6 +45,34 @@ gem install pg -- --with-pg-config=/opt/local/lib/postgresql[version number]/bin
 
 **ArchLinux**: `pacman -S postgresql-libs`
 
+**Windows**:
+```
+1. Download PostgreSQL SDK from official site
+2. Extract archive to postgres-master folder
+3. Copy folder postgres-master/src/interfaces/libpq to v/thirdparty/pg
+4. Copy file postgres-master/src/include/postgres_ext.h to v/thirdparty/pg/libpq
+
+If you build PostgreSQL from source pg_config_ext.h and pg_config.h will be created automatically:
+5. Copy file postgres-master/src/include/pg_config_ext.h to v/thirdparty/pg/libpq
+6. Copy file postgres-master/src/include/pg_config.h to v/thirdparty/pg/libpq
+
+If you do not build PostgreSQL from source code:
+5. Copy file postgres-master/src/include/pg_config_ext.h.in to v/thirdparty/pg/libpq
+- rename pg_config_ext.h.in to pg_config_ext.h
+- in pg_config_ext.h change line **#undef PG_INT64_TYPE** to **#define PG_INT64_TYPE long int**
+6. Copy file postgres-master/src/include/pg_config.h.in to v/thirdparty/pg/libpq
+- rename pg_config.h.in to pg_config.h
+- in pg_config.h change line **#undef PG_VERSION_NUM** to **#define PG_VERSION_NUM X0Y0Z**
+where X is major db version, Y is minor version, Z is patch version. So if your version is 17.1.2
+PG_VERSION_NUM will be 170102, PG_VERSION_NUM should be number, dot sign is changing to 0,
+format *PG_VERSION_NUM 17* without 0 also should work
+
+7. Add libpq.dll to v/thirdparty/pg/win64
+
+To get the libpq.dll file, you can install the PostgreSQL database, 
+and get this dll from its bin/ folder, or compile DB from source code.
+```
+
 ## Getting Started with [PostgreSQL](https://www.postgresqltutorial.com/postgresql-getting-started)
 
 Read this section to learn how to install and connect to PostgreSQL
@@ -54,7 +82,7 @@ Read this section to learn how to install and connect to PostgreSQL
 
 ## Using Parameterized Queries
 
-Parameterized queries (exec_param, etc.) in V require the use of the following syntax: ($n). 
+Parameterized queries (exec_param, etc.) in V require the use of the following syntax: ($n).
 
 The number following the $ specifies which parameter from the argument array to use.
 

vlib/db/pg/pg.c.v

@@ -21,7 +21,7 @@ $if $pkgconfig('libpq') {
 	#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 -I @VEXEROOT/thirdparty/pg/libpq
 	#flag windows -L @VEXEROOT/thirdparty/pg/win64
 }
 
@@ -33,7 +33,11 @@ $if $pkgconfig('libpq') {
 #include <pg_config.h>
 
 // for orm
-#include <arpa/inet.h>
+$if windows {
+	#include <winsock2.h>
+} $else {
+	#include <arpa/inet.h>
+}
 
 #include "@VMODROOT/vlib/db/pg/compatibility.h"