[Enhancement] Append lookup table field to a new output field in LOOKUP command will fail

[LantaoJin]

Is your feature request related to a problem?

In docs/ppl-lang/ppl-lookup-command.md

outputField

• Optional
• Default: <inputField>
• Description: A field of output. You can specify zero or multiple <outputField>. If you specify <outputField> with an existing field name in source query, its values will be replaced or appended by matched values from <inputField>. If the field specified in <outputField> is a new field, in REPLACE strategy, an extended new field will be applied to the results, but fail in APPEND strategy.

Demo

• fact table

id	col1	col2
1	a	b
2	aa	bb
3	null	ccc

• dimension table

id	col1	col3
1	x	y
3	xx	yy

Replace lookup table field to a new output field:

Q1: source=fact | lookup dim id REPLACE col1 as colA

An extended new field colA will be applied to the results

id	col1	col2	colA
1	a	b	x
2	aa	bb	null
3	null	ccc	xx

Append lookup table field to a new output field:

Q2: source=fact | lookup dim id APPEND col1 as colA

Throws with "A column or function parameter with name colA cannot be resolved"

Actually, as the colA is a new field which not existed in fact and dim, the Q2 could be rewritten by Q1.

More complexity, for query

Q3: source=fact | lookup dim id APPEND col1, col3 as colA

could be rewritten to

Q4: source=fact | lookup dim id APPEND col1 | lookup dim id REPLACE col3 as colA

What solution would you like?
The failure is because the current implementation Coalesce expression cannot be resolved with a not existed attribute. To resolve the failure of query Q2 and Q3, we have to extract the output attributes from source plan (not source table).

What alternatives have you considered?
Keep throwing failure since the failed Q2 and Q3 can be rewritten to Q1 and Q4.