Views: if exists / if not exists for DDL

[jepett0]

Description

CREATE VIEW IF NOT EXISTS ...;
DROP VIEW IF EXISTS ...;

Introduce IF ... modifiers for the VIEW DDL to not produce errors if the object already exists / does not exist.

Context

We would like VIEW users to be able to batch create views with scripts (which should not produce errors during the execution) like the following one:

CREATE OR REPLACE VIEW first_view ...;
CREATE OR REPLACE VIEW second_view ...;
CREATE OR REPLACE VIEW third_view ...;
...

However, CREATE OR REPLACE VIEW is a bit harder to implement (it needs to drop the view and recreate it in one transaction + update the server side query cache ☆) than the simple alternative:

DROP VIEW IF EXISTS first_view;
CREATE VIEW first_view ...;

DROP VIEW IF EXISTS second_view;
CREATE VIEW second_view ...;
...

☆

Ideally the server side query cache entry for the queries referencing the view-to-be-dropped should be deleted in the same transaction as the view. However, this feature is not easy to implement (see this PR for more info). I suppose the query cache update lag would be less of an issue for the scripts using DROP VIEW + CREATE VIEW rather than a single CREATE OR REPLACE VIEW, because the later is expected to be atomic.

[UgnineSirdis]

We also need more tests to the following cases:

• DROP VIEW query must fail if there is no such view
• DROP VIEW query must fail if we are attempting to delete view, but it is not a view (topic, table etc)
• DROP VIEW IF EXISTS query must fail if we are attempting to delete view, but it is not a view (topic, table etc)

All these cases should work also for ExecuteQuery calls (not only for ExecuteSchemeQuery) - they are newer API

Also the same cases, but for CREATE type

[jepett0]

Added all 3 DROP VIEW tests you mentioned.

As for the CREATE VIEW, the test that checks that the CREATE VIEW statement does in fact create a view has already existed, so I haven't added it. However, I have added the other two tests for the CREATE VIEW statement.

[github-actions]

⚪ 2024-10-29 11:06:41 UTC Pre-commit check linux-x86_64-relwithdebinfo for 7463682 has started.
⚪ 2024-10-29 11:06:52 UTC Artifacts will be uploaded here
⚪ 2024-10-29 11:10:06 UTC ya make is running...
🟢 2024-10-29 12:14:12 UTC Tests successful.

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
76222	62172	0	0	13952	98

🟢 2024-10-29 12:21:15 UTC Build successful.
🟡 2024-10-29 12:21:34 UTC ydbd size 2.8 GiB changed* by +169.4 KiB, which is >= 100.0 KiB vs main: Warning

ydbd size dash	main: 8acfed4	merge: 7463682	diff	diff %
ydbd size	3 033 991 864 Bytes	3 034 165 320 Bytes	+169.4 KiB	+0.006%
ydbd stripped size	480 683 032 Bytes	480 706 168 Bytes	+22.6 KiB	+0.005%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}

[github-actions]

⚪ 2024-10-29 11:06:51 UTC Pre-commit check linux-x86_64-release-asan for 7463682 has started.
⚪ 2024-10-29 11:07:03 UTC Artifacts will be uploaded here
⚪ 2024-10-29 11:10:12 UTC ya make is running...
🟡 2024-10-29 12:29:10 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
12041	11945	0	29	34	33

🟢 2024-10-29 12:30:09 UTC Build successful.
🟡 2024-10-29 12:30:39 UTC ydbd size 5.7 GiB changed* by +202.4 KiB, which is >= 100.0 KiB vs main: Warning

ydbd size dash	main: 8acfed4	merge: 7463682	diff	diff %
ydbd size	6 139 835 928 Bytes	6 140 043 184 Bytes	+202.4 KiB	+0.003%
ydbd stripped size	1 532 105 648 Bytes	1 532 155 344 Bytes	+48.5 KiB	+0.003%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}