Re: Virtual generated columns

Dean Rasheed <dean.a.rasheed@gmail.com>

From: Dean Rasheed <dean.a.rasheed@gmail.com>

To: Peter Eisentraut <peter@eisentraut.org>

Cc: jian he <jian.universality@gmail.com>, Richard Guo <guofenglinux@gmail.com>, Zhang Mingli <zmlpostgres@gmail.com>, Alexander Lakhin <exclusion@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>

Date: 2025-02-15T12:37:45Z

Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

Expand virtual generated columns for ALTER COLUMN TYPE
- 5069fef1cfae 18.0 landed
Eliminate code duplication in replace_rte_variables callbacks
- 363a6e8c6fcf 18.0 landed
Expand virtual generated columns in the planner
- 1e4351af329f 18.0 landed
Virtual generated columns
- 83ea6c54025b 18.0 landed
Additional tests for stored generated columns
- 41084409f635 18.0 landed
Improve generated_stored test
- 44b61efb7928 18.0 landed
- 86749ea3b766 18.0 landed
Fix handling of CREATE DOMAIN with GENERATED constraint syntax
- 84a67725cd11 18.0 landed
Add pg_constraint rows for not-null constraints
- 14e87ffa5c54 18.0 cited
Put generated_stored test objects in a schema
- 894be11adfa6 18.0 landed
Rename regress test generated to generated_stored
- b9ed4969250d 18.0 landed
Small code simplification
- 7ff9afbbd1df 18.0 landed
Remove useless code
- e26d313bad92 18.0 landed
Remove useless initializations
- da2aeba8f533 18.0 landed
doc: Clarify that pg_attrdef also stores generation expressions
- da486d360103 18.0 landed
Clean out column-level pg_init_privs entries when dropping tables.
- 76618097a6c0 17.0 cited
Re-implement the ereport() macro using __VA_ARGS__.
- e3a87b4991cc 13.0 cited

Attachments

expand-virt-gen-cols-in-planner.patch (text/x-patch) patch

On Fri, 14 Feb 2025 at 10:59, Peter Eisentraut <peter@eisentraut.org> wrote:
>
> On 13.02.25 14:06, jian he wrote:
> > I didn't solve the out join semantic issue.
> > i am wondering, can we do the virtual generated column expansion in
> > the rewrite stage as is,
> > and wrap the expressions in PHVs if the virtual generated
> > columns come from the nullable side of an outer join.
>
> PlaceHolderVar looks like a fitting mechanism for this.  But it's so far
> a planner node, so it might take some additional consideration if we
> want to expand where it's used.

It seems pretty baked into how PHVs work that they should only be
added by the planner, so I think that I agree with Richard -- virtual
generated columns probably have to be expanded in the planner rather
than the rewriter.

> Maybe a short-term fix would be to error out if we find ourselves about
> to expand a Var with varnullingrels != NULL.  That would mean you
> couldn't use a virtual generated column on the nullable output side of
> an outer join, which is annoying but not fatal, and we could fix it
> incrementally later.

I think that would be rather a sad limitation to have. It would be
nice to have this fully working for the next release.

Attached is a rough patch that moves the expansion of virtual
generated columns to the planner. It needs a lot more testing (and
some regression tests), but it does seem to fix all the issues
mentioned in this thread.

Regards,
Dean