Hello. This is my first patch to the project.

 

The patch adds support for system columns in JOIN USING clause.  The problem

can be demonstrated with this code:

 

```sql

CREATE TABLE t  (id int);

CREATE TABLE tt (id int);

 

-- Works:

SELECT * FROM t JOIN tt ON t.xmin = tt.xmin;

-- Doesn't work:

SELECT * FROM t JOIN tt USING (xmin);

```

 

Solution:

1. Use the scanNSItemForColumn() function instead of buildVarFromNSColumn() for

constructing Var objects, as it correctly handles system columns.

2. Remove extra calls to markVarForSelectPriv(), since this function is already

invoked in scanNSItemForColumn().

3. For system columns, collect their negative attribute numbers along with

user-defined column indices into l_colnos and r_colnos.

4. Create a buildVarFromSystemAttribute() function for rebuilding Var objects

with system attributes, analogous to buildVarFromNSColumn(), since

scanNSItemForColumn() is complex and has side effects.

5. Implement a fillNSColumnParametersFromVar() function for building NS columns

with system attributes.

 

Link to PR on GitHub: https://github.com/hilltracer/postgres/pull/3