Horton

Re: Virtual generated columns

Richard Guo <guofenglinux@gmail.com>

View thread
From: Richard Guo <guofenglinux@gmail.com>
To: Zhang Mingli <zmlpostgres@gmail.com>
Cc: Peter Eisentraut <peter@eisentraut.org>, Dean Rasheed <dean.a.rasheed@gmail.com>, Alexander Lakhin <exclusion@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>, jian he <jian.universality@gmail.com>
Date: 2025-02-10T03:54:29Z
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 →

  1. Expand virtual generated columns for ALTER COLUMN TYPE

  2. Eliminate code duplication in replace_rte_variables callbacks

  3. Expand virtual generated columns in the planner

  4. Virtual generated columns

  5. Additional tests for stored generated columns

  6. Improve generated_stored test

  7. Fix handling of CREATE DOMAIN with GENERATED constraint syntax

  8. Add pg_constraint rows for not-null constraints

  9. Put generated_stored test objects in a schema

  10. Rename regress test generated to generated_stored

  11. Small code simplification

  12. Remove useless code

  13. Remove useless initializations

  14. doc: Clarify that pg_attrdef also stores generation expressions

  15. Clean out column-level pg_init_privs entries when dropping tables.

  16. Re-implement the ereport() macro using __VA_ARGS__.

On Sun, Feb 9, 2025 at 7:02 PM Zhang Mingli <zmlpostgres@gmail.com> wrote:
> On Feb 9, 2025 at 16:00 +0800, Alexander Lakhin <exclusion@gmail.com>, wrote:
> Please look at a planner error with a virtual generated column triggered
> by the following script:
> CREATE TABLE t(a int, b int GENERATED ALWAYS AS (a * 1));
>
> SELECT SUM(CASE WHEN t.b = 1 THEN 1 ELSE 1 END) OVER (PARTITION BY t.a)
> FROM t AS t1 LEFT JOIN T ON true;
>
> ERROR:  XX000: wrong varnullingrels (b) (expected (b 3)) for Var 2/1
> LOCATION:  search_indexed_tlist_for_var, setrefs.c:2901

> During the parse stage, we set the Var->varnullingrels in the parse_analyze_fixedparams function.
> Later, when rewriting the parse tree in pg_rewrite_query() to expand virtual columns, we replace the expression column b with a new Var that includes a, since b is defined as a * 1.
> Unfortunately, we overlooked updating the Var->varnullingrels at this point.
> As a result, when we enter search_indexed_tlist_for_var, it leads to a failure.
> While we do have another target entry with the correct varnullingrels, the expression involving the virtual column generates another column reference, which causes the error.
> Currently, I don't have a solid fix.
> One potential solution is to correct the Vars at or after the rewrite stage by traversing the parse tree again using markNullableIfNeeded.
> However, this approach may require exposing the ParseState, which doesn't seem ideal.
> It appears that the virtual column generation function during the rewrite stage does not account for the Var field settings, leading to the errors we are encountering.

Hmm, would it be possible to propagate any varnullingrels into the
replacement expression in ReplaceVarsFromTargetList_callback()?

BTW, I was curious about what happens if the replacement expression is
constant, so I tried running the query below.

CREATE TABLE t (a int, b int GENERATED ALWAYS AS (1 + 1));
INSERT INTO t VALUES (1);
INSERT INTO t VALUES (2);

# SELECT t2.a, t2.b FROM t t1 LEFT JOIN t t2 ON FALSE;
 a | b
---+---
   | 2
   | 2
(2 rows)

Is this the expected behavior?  I was expecting that t2.b should be
all NULLs.

Thanks
Richard