Subject: Re: Adding a stored generated column without long-lived locks

From: Laurenz Albe <[email-masked]>
Date: Tue, 26 May 2026 17:23:31 +0200

On Thu, 2026-05-14 at 15:46 -0700, Alberto Piai wrote:
> > > On Tue Mar 17, 2026 at 5:31 PM +07, Alberto Piai wrote:
> > > 
> > > > I recently needed to add a stored generated column to a table of
> > > > nontrivial size, and realized that currently there is no way to do
> > > > that without rewriting the table under an AccessExclusiveLock.
> 
> The attached v4 is a rebase against current master

I understand the need that the patch fulfills, and I agree that it would be a
nice feature.

I have a few thoughts about this that don't concern the implementation:

1) The SQL standard knows ALTER TABLE ... ADD ... GENERATED ALWAYS AS (...)
   and ALTER TABLE ... ALTER ... DROP EXPRESSION, but there is no provision
   for ALTER TABLE ... ADD GENERATED ALWAYS AS (...).
   So this patch adds non-standard syntax that may one day conflict with
   a new version of the standard.  I think we can still do it, and the
   proposed syntax looks right, but I thought I should mention it.

2) We currently have ALTER TABLE ... ALTER ... SET EXPRESSION AS (...) to
   change the generation expression of a column.  This command always
   rewrites the table, according to the documentation.
   I think that if the present patch adds support to skip rewriting the table
   when a generation expression is added and the expression matches a check
   constraint, changing the generation expression should also be possible
   without a rewrite.  If not, I would consider that a violation of the
   principle of least astonishment.
   Would it be difficult to extend the patch to support that?

3) We already have a couple of tricks to avoid blocking for a long time:

   - ALTER TABLE ... ALTER ... SET NOT NULL can skip the table scan if there
     is a check constraint that makes sure that the column is NOT NULL

   - ALTER TABLE ... ATTACH PARTITION can skip the scan of the new partition
     if there is a check constraint matching the partition constraint

   It would be great to document these little tricks in the documentation,
   probably on the ALTER TABLE page.  This is not necessarily the job of
   this patch, but it would also not be off-topic for the patch.

Comments on the patch:
----------------------

The patch applies and builds cleanly and passes the regression tests.

Missing parts:

- There is no documentation.  At least ALTER TABLE needs a description of the
  new syntax, and would ideally mention the trick with the check constraint.

- There should be support for command line completion for the new syntax.

Bugs:

- The patch doesn't test if the column is an identity column:

  CREATE TABLE tab (id bigint GENERATED ALWAYS AS IDENTITY PRIMARY KEY);
  INSERT INTO tab VALUES (DEFAULT);
  ALTER TABLE tab ALTER id ADD GENERATED ALWAYS AS (1) STORED;

  The ALTER TABLE should fail, but doesn't.

- Strange behavior with sequences owned by the column:

  CREATE TABLE tab (id bigserial);
  INSERT INTO tab VALUES (DEFAULT);
  ALTER TABLE tab ALTER id ADD GENERATED ALWAYS AS (1) STORED;
  \ds tab_id_seq
               List of sequences
   Schema |    Name    |   Type   |  Owner   
  --------+------------+----------+----------
   public | tab_id_seq | sequence | postgres
  (1 row)

  I think that any sequence owned by the column should be dropped.
  Alternatively, you could throw an error.

- Incorrect handling of NULL values:

  CREATE TABLE tab (col1 integer, col2 integer);
  INSERT INTO tab VALUES (2, NULL);
  -- works, because NULL results from the check are accepted
  ALTER TABLE tab ADD CHECK (col2 = col1);

  SELECT pg_relation_filenode('tab');
   pg_relation_filenode 
  ----------------------
                  19920
  (1 row)

  ALTER TABLE tab ALTER col2 ADD GENERATED ALWAYS AS (col1) STORED;
  SELECT pg_relation_filenode('tab');
   pg_relation_filenode 
  ----------------------
                  19920
  (1 row)

  TABLE tab;
   col1 | col2 
  ------+------
      2 |    ∅
  (1 row)

  I am not sure what the correct approach would be.  The simple approach would be
  to only skip the rewrite if the column has a NOT NULL constraint or an equivalent
  check constraint, but perhaps you can think of a way to do better.

Comments on the code:

> --- a/src/backend/commands/tablecmds.c
> +++ b/src/backend/commands/tablecmds.c
> @@ -5093,6 +5102,13 @@ ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,
>             ATSimpleRecursion(wqueue, rel, cmd, recurse, lockmode, context);
>             pass = AT_PASS_SET_EXPRESSION;
>             break;
> +       case AT_AddGeneratedAsExprStored:

You should add a comment, same as for the other branches.

> @@ -6695,6 +6717,8 @@ alter_table_type_to_string(AlterTableType cmdtype)
>             return "ALTER COLUMN ... SET NOT NULL";
>         case AT_SetExpression:
>             return "ALTER COLUMN ... SET EXPRESSION";
> +       case AT_AddGeneratedAsExprStored:
> +           return "ALTER COLUMN ... ADD GENERATED ALWAYS AS (...) STORED";

Keep it short, like "ALTER COLUMN ... ADD GENERATED".

> --- a/src/test/modules/test_ddl_deparse/test_ddl_deparse.c
> +++ b/src/test/modules/test_ddl_deparse/test_ddl_deparse.c
> @@ -129,6 +129,9 @@ get_altertable_subcmdinfo(PG_FUNCTION_ARGS)
>             case AT_SetNotNull:
>                 strtype = "SET NOT NULL";
>                 break;
> +           case AT_AddGeneratedAsExprStored:
> +               strtype = "ADD GENERATED ALWAYS AS (...) STORED";
> +               break;

I suggest "ALTER COLUMN ADD GENERATED ALWAYS AS", but I won't insist.

Yours,
Laurenz Albe

---

From: "Alberto Piai" <[email-masked]>
Date: Wed, 27 May 2026 19:43:53 +0200

Hi,

On Fri May 15, 2026 at 12:46 AM CEST, Alberto Piai wrote:
> On Fri Apr 24, 2026 at 2:10 AM PDT, Alberto Piai wrote:
>> On Tue Apr 7, 2026 at 5:02 PM +08, Alberto Piai wrote:

here's a not-so-brief summary of the conversations around this topic at
pgconf.dev, and a new proposal at the end.

I had the chance to bring this up with other attendees, and many
recognized the use case as a useful one, addressing a real operational
issue.

In particular, I had great feedback from Staš Kotarac Guček, who pointed
out a major flaw in my current proposal: a constraint of the form

  CHECK (c = expr)

would not work correctly when expr evaluates to null for some rows.
Thank you Staš, in the next iteration I will change the constraint to
use IS NOT DISTINCT FROM, instead.

I briefly mentioned this topic to Tom Lane, who quickly replied with the
question: should this not fail when it can't use the constraint, instead
of overwriting the contents of the column?

Thanks Tom, I will get to this later in this mail.

I had registered this patch for the in-person commitfest at pgconf.dev,
and Álvaro Herrera picked it up for review. Thank you Álvaro, and thank
you Peter for organizing the event.

We managed to find some time on the very last day of the conference, and
went through the current design and code. The open items (which I will
address in the next iteration of this patch) are:

* missing user documentation

  I will work on this next. I think it's a good way to explain the
  feature even early during development. I just didn't want to do it
  _too_ early, without having had any feedback.

* try to minimize command counter increments

  There might be one call to CommandCounterIncrement() which is not
  necessary, I'll try remove it.

* comment on why it is necessary to clear missing values when rewriting
  the table

  ATExecAlterColumnType() and ATExecSetExpression() both do this
  explicitly when requesting a table rewrite. I'll extend the comment,
  and also look into whether this is something that should be done any
  time a table rewrite happens. In that case, it might be worth moving
  this into the rewriting code rather than having each caller do it.

* interactions with other subcommands in the same alter table statement

  My reasoning regarding this was: if I do this in
  AT_PASS_SET_EXPRESSION, it should be safe. I will invest some more
  time into this and add tests, too.

We also looked at the overall design of the new command, and we agreed
that it is a fitting addition to our current SET EXPRESSION and DROP
EXPRESSION. Regarding the question of whether it should be SET or ADD,
we agreed that ADD (i.e. the current proposal) is clearer, especially
for its similarity to ADD GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY.

Regarding the question of "should this fail or rewrite the table when a
usable constraint isn't found": Álvaro's suggestion here was to use a
more ad-hoc command, meant more specifically for this use case of
converting into a stored generated column without rewriting it. If the
command would be dedicated specifically to this, it would make sense to
have it fail when a usable constraint isn't found.

Last but not least, I also discussed this with Laurenz Albe, and he
wrote a very useful review in this thread. I will address that
separately and reply directly to that mail, but one point I can already

>    proposed syntax looks right, but I thought I should mention it.

I'd like to take his point, together with the question from Tom and the
suggestion by Álvaro, and make a new proposal for the design of this
command.

Design iteration 2
------------------

Syntax:

  ALTER TABLE t ALTER COLUMN c
    ADD GENERATED ALWAYS AS (expr) STORED USING CONSTRAINT check_name

check_name must be a valid constraint of the form

  CHECK (c IS NOT DISTINCT FROM (expr))

This fails if:

- a check constraint named check_name is not found for table c
- the constraint is not valid
- the constraint does not match exactly the expr the user intends to use
  as a stored default expression

On success, the table c is now a stored generated column with the given
default expression, and the check_name constraint has been removed.

This addresses Tom's remark, we can now fail instead of just rewriting
the column.

It improves slightly upon the issue of a potential conflict with a
future edition of the SQL standard, by being more specific. I don't see
a way to be completely sure we won't have conflicts. We could improve
more by making the syntax more "alien" and very unlinkely to be picked
up by the standard, but at a usability cost for Postgres. I'm open to
suggestions.

It improves upon another question raised by Álvaro: does the user have
to clean up the constraint? In v1 I felt it was better to have the user
remove it after the migration. Since here it's explicitly mentioned as
the constraint to use to migrate the column, I think it's OK to remove
it. We are conceptually moving it from being a constraint to being the
new default expression.

The implementation should also be simpler, since there will never be a
table rewrite.

Any thoughts about this?

Best regards,

Alberto

-- 
Alberto Piai
Sensational AG
Zürich, Switzerland

---

From: "Alberto Piai" <[email-masked]>
Date: Wed, 27 May 2026 19:44:23 +0200

>    proposed syntax looks right, but I thought I should mention it.

Thank you for bringing this up. I don't have access to the standard, but
the chance of a possible conflict with future editions was at the back
of my mind. I don't see a way to exclude it completely. In a sibling
mail in this thread (you should be in CC), I have made a new iteration
on this proposal, which also tries to make the command more specific to
avoid future conflicts.

>    Would it be difficult to extend the patch to support that?

Yes, I don't see a way to make that work. Since we're talking only about
stored values, a rewrite will always be necessary. However, using this
new command, a user could add a column with the new expression, then
atomically drop the old one and rename. All without holding onto an
AccessExclusiveLock for a long time :)

>    this patch, but it would also not be off-topic for the patch.

The SET NOT NULL one and the ATTACH PARTITION one are documented in the
section specific to the command. However

  or, if an equivalent index already exists, it will be attached to the
  target table's index, as if ALTER INDEX ATTACH PARTITION had been
  executed

is not very explicit about the advantages this has for online
migrations.

In the NOTES section of the ALTER TABLE page, there is a paragraph about
NOT VALID / VALIDATE, which is another operation in the same spirit as
this.

Maybe we could group them all in a new section dedicated to online
schema migrations?

(However, even if it's definitely on-topic with this patch, I would work
on this in a separate patch / email thread.)

>   new syntax, and would ideally mention the trick with the check constraint.

Yes, I will work on this next. I also believe it's a great way to show a
feature, even early during development. I just wanted to avoid doing it
_too_ early, before having had any feedback about the idea.

> - There should be support for command line completion for the new syntax.

Great idea, I'll add this too.

>    Schema |    Name    |   Type   |  Owner   

Thanks for testing this!

I have reused RememberAllDependentForRebuilding() which does some
validation, but was originally meant for ALTER COLUMN TYPE. I will add
checks and tests for these cases, but to be consistent with how the
other dependencies are handled, I think it's better to throw an error
here (this is what happens for example if trying to ALTER TYPE of a
column used by a function).

> - Incorrect handling of NULL values:

See sibling mail, in the next iteration the constraint will have to use
IS NOT DISTINCT FROM. I think that should cover all cases.

> I suggest "ALTER COLUMN ADD GENERATED ALWAYS AS", but I won't insist.

Agreed, will fix all these in the next version of the patch.

Thank you again for the review!

Best regards,

Alberto

-- 
Alberto Piai
Sensational AG
Zürich, Switzerland

---

From: Laurenz Albe <[email-masked]>
Date: Mon, 15 Jun 2026 20:38:55 +0200

> suggestion by Álvaro, and make a new proposal for the design of this

I like this proposal.  It avoids the question "to rewrite or not to
rewrite" by just outright failing if there is no suitable constraint.

The idea to avoid the problem with NULL by forcing IS NOT DISTINCT FROM
in the constraint is a good solution.  Perhaps you could also allow
the equality operator if the column in question is defined NOT NULL.

> up by the standard, but at a usability cost for Postgres. I'm open to

I don't think that the new proposal makes it less likely to get in
conflict with later additions to the standard.  But I don't think that
inventing unlikely syntax to avoid such conflicts is a good idea.
First, the standard committee itself seems (or seemed) to have a strong
predilection for alien, verbose syntax.
Second, if we end up with weird, unwieldy syntax, that would be bad.

No, the syntax you are proposing sounds reasonable to me.

> The implementation should also be simpler, since there will never be a

Yes.  I think that you should not drop the constraint.  That's what I'd
expect, similar to how we don't drop the check constraint that allows
to skip the table scan in ALTER TABLE ... ALTER COLUMN ... SET NOT NULL
or ALTER TABLE ... ATTACK PARTITION.

I feel that automatically dropping the constraint is a bit too much
black magic, but it is more a feeling than a conviction.

Yours,
Laurenz Albe

---

From: Laurenz Albe <[email-masked]>
Date: Mon, 15 Jun 2026 22:41:15 +0200

> AccessExclusiveLock for a long time :)

With your new proposal to never rewrite the table, but fail instead if
there is no constraint, my objection loses its point, so I withdraw it.

> Maybe we could group them all in a new section dedicated to online

You are right, the existing shortcuts are documented.  Your new proposal
makes the proposed feature different from these existing cases, so I don't
think lumping them together is a good idea now.

> Agreed, will fix all these in the next version of the patch.

Great; I'm looking forward to it.

Yours,
Laurenz Albe

---

From: Alberto Piai <[email-masked]>
Date: Tue, 30 Jun 2026 15:44:28 +0200

> the equality operator if the column in question is defined NOT NULL.

In the attached v5 patch I've implemented this design, and went one step
further (let me know what you think). While discussing this with my
colleagues at work, the question came up (thanks, Philip!): now that we
mention the constraint explicitly, what's the point of repeating the
expression too? The constraint already defines an equality to an
expression. I think this is a very good point, and it removes one
further way in which the operation could fail,  so I went ahead and
changed the command to not mention the expression. It takes the
expression defined in the constraint and uses _that_ as the generator
expression of the column.

Design iteration 3
------------------

Syntax:

  ALTER TABLE t ALTER COLUMN c
  ADD GENERATED ALWAYS STORED USING CONSTRAINT check_name

check_name must be a valid constraint of a specific shape. If the
column is nullable:

  CHECK (c IS NOT DISTINCT FROM expr)

If the column is NOT NULL, either of the following is acceptable:

  CHECK (c = expr)
  CHECK (c IS NOT DISTINCT FROM expr)

The column is then changed to be a stored generated column, with the
"expr" from the constraint as its generator expression.

> to skip the table scan in ALTER TABLE ... ALTER COLUMN ... SET NOT
> NULL or ALTER TABLE ... ATTACK PARTITION.
>

I don't have a strong opinion on whether to cleanup or not, I'll gladly
take your input. This version of the patch does not drop the constraint
anymore.

This version addresses your inputs from the last review:

- I added documentation for the new alter table form to alter_table.sgml
- Tab completion for psql is there now
- The missing error conditions in case of an identity column or
  sequences are now handled, more about this in the next section.

Failure conditions
------------------

There's quite a few invalid states that cannot be reached via CREATE
TABLE and should not be reachable via ALTER TABLE either.

The following are detected and fail the operation:

- the column is already a generated column

- the column is an identity column

- the column is referenced by a sequence (it is most likely a serial
  column)

- the column is referenced by another column's default expression

- the column references another generated column

- the column is referenced in a partition key, either directly or
  through a whole row expression

- the new default expression is not immutable

Additionally, we of course bail if the constraint is not found, not
valid, not enforced or doesn't match the specific structure we need.

Another case I considered is the column being referenced in the body of
a pre-parsed function (BEGIN ATOMIC SQL functions). In this case though,
it seems to me that we don't need to fail here: we are not altering the
type of the column, and when reading a stored generated column there's
no expression replacement happening (as it does when reading virtual
columns).

Partitioning/inheritance is supported only on the whole hierarchy at
once (see 8bf6ec3ba3a44448817af47a080587f3b71bee08). Trying to change
the column at only one level will fail, as well as any of the subtrees.

I also added a test to explictily check that we're not accidentally
enqueuing a table scan for verification in phase 3, as avoiding this
kind of work is the whole point of the command.

Logical replication
-------------------

The interaction with logical replication is tricky, since a publication
can have the option to publish generated columns or not (which is the
default).

When not publishing stored generated columns, inserts or updates would
be replicated while backfilling, and would then suddenly stop when the
column is turned into a stored generated column.

One way to avoid this is to set up triggers on the subscriber too,
before altering the column on the publisher. This way updates and
inserts would not lose the column's value on the subscriber, which can
then be migrated by using the new alter table command.

When publishing stored generated columns instead, it is not possible to
have the same column be stored generated on both the publisher and the
subscriber (see Table 29.2 in section 29.6. Generated Column
Replication). The only supported configuration has a regular column on
the side of the subscriber. (Note that this is not specific to this new
command.)

This makes this scenario a lot easier: the column is migrated on the
publisher only, and the subscriber won't lose any value.

To test these two scenarios, I wrote TAP tests for the subscription
suite. However, I'm inclined to not add them to the test suite. I have
attached them to this email separately.

Other changes since v4
----------------------

I have changed phase 2 to be ran at AT_PASS_ADD_OTHERCONSTR, before it
was at AT_PASS_SET_EXPRESSION. The reason to do it there was to reuse
the cleanup steps in ATPostAlterTypeCleanup when a table rewrite did
happen. But since now never rewrite, this is not necessary anymore.

Looking forward to your thoughts on this!

Alberto

-- 
Alberto Piai
Sensational AG
Zürich, Switzerland

---

From: Alberto Piai <[email-masked]>
Date: Fri, 3 Jul 2026 08:42:18 +0200

v5 had a rather brittle test that was relying on a DEBUG message being
emitted when phase 3 verifies or rewrites a table (via
client_min_messages). That was failing on the CI because the output
depends on any other setting that might affect logging (in this case it
was log_statement).

The attached v6 replaces it with an injection_point test. If adding new
injection points to detect scans/rewrites is considered too much, I can
back it out and set/reset log_statement too. But I do prefer the
injection_point test.

If the new injection point is good, there are also a couple more tests
which might be updated to use it.

Regards,

Alberto

-- 
Alberto Piai
Sensational AG
Zürich, Switzerland

---

From: Laurenz Albe <[email-masked]>
Date: Tue, 07 Jul 2026 20:16:36 +0200

> expression of the column.

I agree with that idea, it shortend the syntax and leaves less room for
mistakes.

The syntax you ended up with (ADD GENERATED ALWAYS STORED USING CONSTRAINT)
is ugly as hell.  I see your point in having ALWAYS and STORED, but perhaps
ADD GENERATED ALWAYS USING CONSTRAINT ... STORED would be better, as it is
syntactically more like GENERATED ALWAYS AS (...) STORED, which would make
it easier to remember.

Or perhaps ADD GENERATED USING CONSTRAINT would be enough, since ALWAYS and
STORED are the only possible choice anyway.  I am a bit uncertain on what
is best here.

> take your input. This version of the patch does not drop the constraint

I like it that way, because ALTER TABLE ATTACH PARTITION and ALTER TABLE
ALTER COLUMN SET NOT NULL don't drop the constraint they use either.
But the case is not exactly the same, so I won't insist.

>   sequences are now handled, more about this in the next section.

Thanks.

I think you didn't adapt the documentation sufficiently after you dropped
the generation expression from the syntax:

> +      This form changes a regular column into a stored generated column, using
> +      the expression from the given constraint. The constraint must be a
> +      <literal>CHECK</literal> constraint proving that the values of the
> +      column already satisfy the generation expression. The operation will
> +      then be performed without rewriting the table.

The "generation expression" suddenly surfaces towards the end of the paragraph
and makes the reader wonder where it comes from.

Perhaps:

  This form changes a regular column into a stored generated column, using
  the expression from the given check constraint as generation expression.
  The operation will be performed without rewriting the table, which avoids
  holding an <literal>ACCESS EXCLUSIVE</literal> lock for a longer time.

That would render the immediately following paragraph unnecessary.

The patch applies, builds and passes the regression tests.

There is a weird asymmetry in that the order in which you write the check
constraint matters:

  CREATE TABLE tab (a integer PRIMARY KEY, b integer NOT NULL);
  INSERT INTO tab VALUES (1, 2);

  -- this fails
  ALTER TABLE tab ADD CONSTRAINT c CHECK (2 * a = b);
  ALTER TABLE tab ALTER b ADD GENERATED ALWAYS STORED USING CONSTRAINT c;
  ERROR:  cannot convert a column into a stored generated column without a constraint to prove that the values are consistent
  DETAIL:  could not find a valid constraint "c" CHECK ("b" = expr) or CHECK ("b" IS NOT DISTINCT FROM (expr))

  -- but this works
  ALTER TABLE tab DROP CONSTRAINT c;
  ALTER TABLE tab ADD CONSTRAINT c CHECK (b = 2 * a);
  ALTER TABLE tab ALTER b ADD GENERATED ALWAYS STORED USING CONSTRAINT c;

I think that both variants should be accepted, but I am not certain.

The following error message is not very helpful:

  CREATE TABLE tab (
    a integer DEFAULT 2,
    b integer
      CONSTRAINT con CHECK (b IS NOT DISTINCT FROM 2 + random())
  );

  ALTER TABLE tab ALTER b ADD GENERATED ALWAYS STORED USING CONSTRAINT con;
  ERROR:  cannot convert a column into a stored generated column without a constraint to prove that the values are consistent
  DETAIL:  could not find a valid constraint "con" CHECK ("b" IS NOT DISTINCT FROM (expr))

Perhaps it would be better to proceed in three steps:

- find a check constraint with the given name
- verify that the check constraint has the correct shape
- verify that the expression is immutable

Each step could have a different, helpful error message.

I looked at the code too, and I could only spot smaller problems:

> [...]
> +ATPrepAddExpressionStored(Relation rel,
> +                         AlterTableCmd *cmd,
> +                         bool recurse, bool recursing,
> +                         LOCKMODE lockmode)
> [...]
> +   /*
> +    * Cannot change only inherited columns to be stored generated columns.
> +    */
> +   if (!recursing)
> +   {
> [...]
> +       if (attTup->attinhcount > 0)
> +           ereport(ERROR,
> +                   (errcode(ERRCODE_INVALID_TABLE_DEFINITION),
> +                    errmsg("cannot change inherited column to be a stored generated column")));
> +   }

The comment is cryptic, and I had to read the code to understand what you mean.
Perhaps:

/* convert inherited columns only if the entire hierarchy is changed */

> +/*
> + * Detect dependencies which should stop us from turning a regular column
> + * into a stored generated column.
> + */
> +static void
> +checkDependenciesForAddExprStored(Relation rel,
> +                                 AttrNumber attnum,
> +                                 const char *colName)
> [...]
> +       switch (foundObject.classId)
> +       {
> +           case RelationRelationId:
> +               {
> +                   char        relKind = get_rel_relkind(foundObject.objectId);
> +
> +                   if (relKind == RELKIND_SEQUENCE)
> +                       ereport(ERROR,
> +                               (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
> +                                errmsg("cannot convert a serial column to a stored generated column"),
> +                                errdetail("\"%s\" of relation \"%s\"  depends on sequence %s",
> +                                          colName, RelationGetRelationName(rel),
> +                                          getObjectDescription(&foundObject, false))));

There is an extra space in the error message.

> +                   break;
> +               }
> +           case AttrDefaultRelationId:
> +               {
> +                   ObjectAddress col = GetAttrDefaultColumnAddress(foundObject.objectId);
> +
> +                   if (col.objectId == RelationGetRelid(rel) &&
> +                       col.objectSubId == attnum)
> +                   {
> +                       /*
> +                        * Ignore the column's own default expression. We
> +                        * handle sequences above, and for a column which is
> +                        * already a generated column we should never get
> +                        * here.
> +                        */

It's not strictly required, but it would be great if you could run pgindent
to get comments and other parts of the code formatted properly.

> +                   }
> +                   else
> +                   {
> +                       ereport(ERROR,
> +                               (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
> +                                errmsg("cannot convert a column referenced in a default expression to a stored generated column"),
> +                                errdetail("Column \"%s\" is referenced by generated column \"%s\".",
> +                                          colName,
> +                                          get_attname(col.objectId, col.objectSubId, false))));

This is confusing me.
The error message seems to suggest that you cannot use a generated column in a
DEFAULT expression. But you can never use other columns in a default expression
anyway, right?
The detail message says something different, namely that the column is referenced
my a generated column (do you mean it is used in the generation expression)?

I believe that if I get confused by the error message, the average user will
also get confused.

> +                   }
> +                   break;
> +               }
> +           default:
> +               /* We're not interested in the row */
> +               break;
> +       }

Perhaps a better comment would be

/* other dependencies, e.g. by views, are no problem */

> +/*
> + * Subroutine for ATExecAddExpressionStored, used to find a CHECK constraint
> + * to prove that the column values statisfy what will be the generator
> + * expression.
> + *
> [...]
> + *
> + * If a valid constraint is found, this returns both the Oid of the constraint
> + * and the unpacked expression.
> + */
> +static Node *
> +findUsableConstraintForAddExprStored(Relation rel, AttrNumber attnum,
> +                                    bool attisnotnull,
> +                                    const char *conname)

I see that it returns a Node, not an Oid and an expression.
If the Oid of the constraint is actually returned somewhere inside the
"Node", the comment should be more specific about it.

> +static ObjectAddress
> +ATExecAddExpressionStored(AlteredTableInfo *tab,
> +                         Relation rel,
> +                         const char *colName,
> +                         Constraint *def)
> +{
> [...]
> +   if (attTup->attidentity)
> +       ereport(ERROR,
> +               (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
> +                errmsg("Cannot convert an identity column to a stored generated column"),
> +                errdetail("column \"%s\" of relation \"%s\" is an identity column",
> +                          colName, RelationGetRelationName(rel))));

Message style: the main error message should start with a lower case character,
and the detail message should be a whole sentence (initial capitalization, period).
Error messages should try not to exceed 80 characters (no problem with that here),

> +   if (has_partition_attrs(rel, colRefs, &is_expr))
> +       ereport(ERROR,
> +               (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
> +                errmsg("cannot convert a column into a stored generated column if it's referenced by a partition key"),
> +                errdetail("column \"%s\" is part of the partition key of relation \"%s\"",
> +                          colName, RelationGetRelationName(rel))));

Same as above, plus the error message is too long.
Perhaps:

  ERROR:  cannot convert a column used in a partitioning key to a generated column

I don't think we need to say "stored" everywhere.

> +   if (foundConstraintExpr == NULL)
> +       ereport(ERROR,
> +               (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
> +                errmsg("cannot convert a column into a stored generated column without a constraint to prove that the values are consistent"),
> +                attTup->attnotnull ?
> +                errdetail("could not find a valid constraint \"%s\" CHECK (\"%s\" = expr) or CHECK (\"%s\" IS NOT DISTINCT FROM (expr))",
> +                          def->conname,
> +                          colName,
> +                          colName) :
> +                errdetail("could not find a valid constraint \"%s\" CHECK (\"%s\" IS NOT DISTINCT FROM (expr))",
> +                          def->conname,
> +                          colName)));

Again, the message must be shorter, and perhaps a hint would be better than a detail:

  ERROR:  cannot find a check constraint to prove that the column values are correct
  HINT:  The constraint must be CHECK ("%s" = expr) or CHECK ("%s" IS NOT DISTINCT FROM expr).

I talked about this error message in the beginning.  It is thrown whenever
findUsableConstraintForAddExprStored() returns nothing, which is a bit too unspecific.

Perhaps you could have findUsableConstraintForAddExprStored() throw the errors
instead, then they could be more specific and pertinent.

I don't usually mention that, but since you are a new contributor and explicitly
asked several people for a review (which is fine!): it is expected that you also
review other's patches in the commitfest.

Yours,
Laurenz Albe