feature: support logical view

[Veeupup]

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Summary

• support logical view:

code base:

• create
• alter
• drop
• read from view

docs

• docs

test

• stateless-test

Changelog

• New Feature

Related Issues

Fixes #4610

Test Plan

Unit Tests

Stateless Tests

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[mergify]

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[sundy-li]

Almost looks good to me, missing some stateless tests .

[Veeupup]

Almost looks good to me, missing some stateless tests .

ok, I'll write some stateless tests when alter is finished.

[b41sh]

need to add some docs for View.

[leiysky]

What will happen when a table depended by a view get dropped?

[Veeupup]

it acts as Clickhouse, view just converts query from view table into subquery.
so, if this happens, it'll give error messages just like Clickhouse to tell that the subquery's table not exists. cc @leiysky

# in clickhouse
:) create view view_t1 as select a from t1;
:) drop table t1;
:) select * from view_t1;

SELECT *
FROM view_test

Query id: d3101f4a-2f80-4662-96aa-d3295172bc45


0 rows in set. Elapsed: 0.006 sec.

Received exception from server (version 21.12.1):
Code: 60. DB::Exception: Received from 0.0.0.0:9000. DB::Exception: Table default.t1 doesn't exist. (UNKNOWN_TABLE)

# in databend
databend :) create view view_t1 as select * from t1;
databend :) drop table t1;
databend :) select * from view_t1;

SELECT *
FROM view_t1

Query id: 87fb4594-bd23-4c34-856f-f53f8e6a878b
....
0 rows in set. Elapsed: 0.012 sec.

Received exception from server (version 2021.5.0):
Code: 1025. DB::Exception: Received from 0.0.0.0:9000. DB:Exception. Unknown table 't1'. ()

[sundy-li]

If Alter/Drop happens to the dependent table, the query from the view may fail.

Users should recreate the view manually.

[leiysky]

it acts as Clickhouse, view just converts query from view table into subquery.

Make sense.

[Veeupup]

review resolved. cc @sundy-li @b41sh

[sundy-li]

/LGTM now

[b41sh]

/LGTM