fixed bug 3879, after pushing down project (through the union), we merge multiple projects directly.
fixed bug 3879, after pushing down project (through the union), we merge
multiple projects directly.

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb