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From: Chao Li <[email protected]>
To: David Rowley <[email protected]>
Cc: Tom Lane <[email protected]>
Cc: Peter Eisentraut <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: Postgres hackers <[email protected]>
Subject: Re: Fix tuple deformation with virtual generated NOT NULL columns
Date: Thu, 18 Jun 2026 10:35:37 +0800
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAApHDvq6FaC441C=G4Q9fNZQ-OKmaeW8yP0YjHpjyjBeWhj3+g@mail.gmail.com>
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> On Jun 18, 2026, at 07:37, David Rowley <[email protected]> wrote:
> 
> On Thu, 18 Jun 2026 at 11:06, David Rowley <[email protected]> wrote:
>> Just starting to look now, but I suspect that this code in
>> llvmjit_deform.c needs to be updated now that we have virtual
>> generated columns.
> 
> I've not fully processed all this code yet. I've still not worked out
> why the loop that sets guaranteed_column_number doesn't break when it
> finds something non-guaranteed.
> 
> Here's a draft patch I was experimenting with. It seems to fix the
> issue. I need to spend more time to check it's correct.
> 
> David
> <fix_jit_deform_for_virtual_generated_cols.patch>

I was not aware of the JIT code before. I think the good thing is that the new test uncovered this JIT bug.

I tested the fix, and it seems to work. While tracing the code, I wondered about this part:
```
-		if (att->attnullability == ATTNULLABLE_VALID &&
-			!att->atthasmissing &&
-			!att->attisdropped)
+		if (attr->attgenerated == ATTRIBUTE_GENERATED_VIRTUAL)
+			break;
+
+		if (catt->attnullability == ATTNULLABLE_VALID &&
+			!catt->atthasmissing &&
+			!catt->attisdropped)
 			guaranteed_column_number = attnum;
```

When computing guaranteed_column_number, I think we can just skip the virtual generated column rather than break. Using the test from Tom’s email:
```
CREATE TABLE gtest21c (a int NOT NULL, b int GENERATED ALWAYS AS (a * 2) VIRTUAL NOT NULL, c int NOT NULL);
```

Here, “c" is a real NOT NULL column, so we can skip “b" and let “c" set guaranteed_column_number. This matters for the later check:
```
	/*
	 * Check if it is guaranteed that all the desired attributes are available
	 * in the tuple (but still possibly NULL), by dint of either the last
	 * to-be-deformed column being NOT NULL, or subsequent ones not accessed
	 * here being NOT NULL.  If that's not guaranteed the tuple headers natt's
	 * has to be checked, and missing attributes potentially have to be
	 * fetched (using slot_getmissingattrs().
	 */
	if ((natts - 1) <= guaranteed_column_number)
	{
```

If we break at “b", then this check goes to the else branch and invokes the maxatt check plus slot_getmissingattrs(), even though the later NOT NULL “c" proves that the tuple has enough attributes.

Otherwise, the fix looks good to me.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/

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