Re: Virtual generated columns

in v7.

doc/src/sgml/ref/alter_table.sgml
<phrase>and <replaceable
class="parameter">column_constraint</replaceable> is:</phrase>

section need representation of:
GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) [VIRTUAL]

in RelationBuildTupleDesc(Relation relation)
we need to add "constr->has_generated_virtual" for the following code?

if (constr->has_not_null ||
constr->has_generated_stored ||
ndef > 0 ||
attrmiss ||
relation->rd_rel->relchecks > 0)

also seems there will be table_rewrite for adding virtual generated
columns, but we can avoid that.
The attached patch is the change and the tests.

i've put the tests in src/test/regress/sql/fast_default.sql,
since it already has event triggers and trigger functions, we don't
want to duplicate it.