Re: Virtual generated columns

statistic related bug.
borrow examples from
https://www.postgresql.org/docs/current/sql-createstatistics.html

CREATE TABLE t3 (a timestamp PRIMARY KEY, b timestamp GENERATED
ALWAYS AS (a) VIRTUAL);
CREATE STATISTICS s3 (ndistinct) ON b FROM t3;
INSERT INTO t3(a) SELECT i FROM generate_series('2020-01-01'::timestamp,
'2020-12-31'::timestamp,
'1 minute'::interval) s(i);
ANALYZE t3;
CREATE STATISTICS s3 (ndistinct) ON date_trunc('month', a),
date_trunc('day', b) FROM t3;
ANALYZE t3;
ERROR: unexpected virtual generated column reference

--this is allowed
CREATE STATISTICS s5 ON (b + interval '1 day') FROM t3;
--this is not allowed. seems inconsistent?
CREATE STATISTICS s6 ON (b ) FROM t3;

in CreateStatistics(CreateStatsStmt *stmt)
we have

if (selem->name)
{
if (attForm->attgenerated == ATTRIBUTE_GENERATED_VIRTUAL)
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("statistics creation on virtual
generated columns is not supported")));
}
else if (IsA(selem->expr, Var)) /* column reference in parens */
{
if (get_attgenerated(relid, var->varattno) ==
ATTRIBUTE_GENERATED_VIRTUAL)
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
errmsg("statistics creation on virtual
generated columns is not supported")));
}
else /* expression */
{
...
}

you didn't make sure the last "else" branch is not related to virtual
generated columns