Re: Virtual generated columns

On 05.09.24 10:27, jian he wrote:
> On Wed, Sep 4, 2024 at 4:40 PM Peter Eisentraut <peter(at)eisentraut(dot)org> wrote:
>>
>>
>> Here is an implementation of this. It's much nicer! It also appears to
>> fix all the additional test cases that have been presented. (I haven't
>> integrated them into the patch set yet.)
>>
>> I left the 0001 patch alone for now and put the new rewriting
>> implementation into 0002. (Unfortunately, the diff is kind of useless
>> for visual inspection.) Let me know if this matches what you had in
>> mind, please. Also, is this the right place in fireRIRrules()?
>
> hi. some minor issues.
>
> in get_dependent_generated_columns we can
>
> /* skip if not generated column */
> if (!TupleDescAttr(tupdesc, defval->adnum - 1)->attgenerated)
> continue;
> change to
> /* skip if not generated stored column */
> if (!(TupleDescAttr(tupdesc, defval->adnum -
> 1)->attgenerated == ATTRIBUTE_GENERATED_STORED))
> continue;

I need to study more what to do with this function. I'm not completely
sure whether this should apply only to stored generated columns.

> in ExecInitStoredGenerated
> "if ((tupdesc->constr && tupdesc->constr->has_generated_stored)))"
> is true.
> then later we finish the loop
> (for (int i = 0; i < natts; i++) loop)
>
> we can "Assert(ri_NumGeneratedNeeded > 0)"
> so we can ensure once has_generated_stored flag is true,
> then we should have at least one stored generated attribute.

This is technically correct, but this code isn't touched by this patch,
so I don't think it belongs here.

> similarly, in expand_generated_columns_internal
> we can aslo add "Assert(list_length(tlist) > 0);"
> above
> node = ReplaceVarsFromTargetList(node, rt_index, 0, rte, tlist,
> REPLACEVARS_CHANGE_VARNO, rt_index, NULL);

Ok, I'll add that.

> @@ -2290,7 +2291,9 @@ ExecBuildSlotValueDescription(Oid reloid,
> if (table_perm || column_perm)
> {
> - if (slot->tts_isnull[i])
> + if (att->attgenerated == ATTRIBUTE_GENERATED_VIRTUAL)
> + val = "virtual";
> + else if (slot->tts_isnull[i])
> val = "null";
> else
> {
> Oid foutoid;
> bool typisvarlena;
> getTypeOutputInfo(att->atttypid, &foutoid, &typisvarlena);
> val = OidOutputFunctionCall(foutoid, slot->tts_values[i]);
> }
>
> we can add Assert here, if i understand it correctly, like
> if (att->attgenerated == ATTRIBUTE_GENERATED_VIRTUAL)
> {
> Assert(slot->tts_isnull[i]);
> val = "virtual";
> }

Also technically correct, but I don't see what benefit this would bring.
The code guarded by that assert would not make use of the thing being
asserted.