release-22.2: sql/catalog/schemaexpr: fix bug when new column default has different…

[ajwerner]

Backport 1/1 commits from #95398.

/cc @cockroachdb/release

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… type

Since 22.2 we permit default expressions to contain types which are not exactly the same as the column type; it is valid to have an expression which can be cast to the column type in an assignment context. Generally, the optimizer handles inserting the assignment cast into the execution of the relevant mutations. Unfortunately, the cast was not present for backfills.

This PR detects the situation where a cast is needed and insert it directly into the plan of the backfill (or import).

Epic: None

Fixes: #93398

Release note (bug fix): Since 22.2, default expressions can have a type which differs from the type of the column so long as the expression's type can be cast in an assignment context. Unfortunately, this code had a bug when adding new columns. The code in the backfill logic was not sophisticated enough to know to add the cast; when such a default expression was added to a new column it would result in a panic during the backfill. This bug has now been fixed.

Release justification: Fixes a bug with newly introduced feature.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[mgartner]

Merging this so I can put the backport for #95459.