Online DDL: ALTER VIEW and INSTANT migrations do not respect ddl strategy's --postpone-completion

[shlomi-noach]

When submitting for example this Online DDL, and assuming view v exists:

vtctlclient -- ApplySchema --skip_preflight --ddl_strategy "online  --allow-zero-in-date --allow-concurrent --postpone-completion" --sql "alter view v as select 17 from dual" commerce

the scheduler runs the statement to immediate completion, even though --postpone-completion is specified.

This happens because originally we were only dealing with tables. We only considered CREATE and DROP statements to be "immediate", and assumed that all ALTER statements are handled by gh-ost or vreplication. But an ALTER VIEW is an immediate operation, too, and this needs to be taken into consideration.

The new (and still undocumented --prefer-instant-ddl) flag also does not respect --postpone-completion. For example:

vtctlclient -- ApplySchema --skip_preflight --ddl_strategy "online  --allow-zero-in-date --allow-concurrent --postpone-completion --prefer-instant-ddl" --sql "alter table corder add column i2 int not null default 0" commerce

Here, the problem is that the query is sent to run, but it's only at that stage that it is processed and evaluated, and found to be eligible to INSTANT ddl.