Re: Doc: fix the rewrite condition when executing ALTER TABLE ADD COLUMN

On Wed, Apr 30, 2025 at 5:15 AM Peter Eisentraut <peter(at)eisentraut(dot)org> wrote:
>
> On 28.04.25 18:56, Álvaro Herrera wrote:
> > On 2025-Apr-23, Nathan Bossart wrote:
> >
> >> On Mon, Mar 24, 2025 at 11:37:20AM +0100, Álvaro Herrera wrote:
> >
> >>> I'd add a note about these two things to the open items page, and wait
> >>> to see if we get some of these limitations fixed, so that if we don't,
> >>> we remember to note this limitation in the documentation.
> >>
> >> Are we still waiting on something for this, or should we proceed with the
> >> documentation changes? It doesn't seem tremendously urgent, but I noticed
> >> it's been about a month since the last message on this thread.
> >
> > I've edited the Open Items page to disclaim my responsibility from this
> > item, since this comes from virtual generated columns which is not my
> > turf. I think we should just document the current state of affairs; we
> > can come back with further code improvements during the next cycle.
>
> Here is a proposed patch that includes some text about virtual generated
> columns and also fixes up a small mistake in the previous patch
> (confused identity and generated columns) and improves the wording and
> formatting a bit more.

If I were going to quibble, I'd probably rewrite the second paragraph as

+ Changing the type of an existing column will normally cause the
entire table
+ and its indexes to be rewritten.
+ As an exception, when changing the type of an existing column,
if the <literal>USING</literal> clause does not change the column
contents and the old type is either binary coercible to the new type
or an unconstrained domain over the new type, a table rewrite is not
- needed. However, indexes must always be rebuilt unless the system
+ needed. However, indexes will still need to be rebuilt unless the system
can verify that the new index would be logically equivalent to the
existing one. For example, if the collation for a column has been
changed, an index rebuild is required because the new sort
order might be different. However, in the absence of a collation
change, a column can be changed from <type>text</type> to
<type>varchar</type> (or vice versa) without rebuilding the indexes
- because these data types sort identically. Table and/or index
+ because these data types sort identically.

But otherwise this LGTM.

Robert Treat
https://xzilla.net