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Clarify behavior of adding and altering a column in same ALTER command.

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The behavior of something like

ALTER TABLE transactions
  ADD COLUMN status varchar(30) DEFAULT 'old',
  ALTER COLUMN status SET default 'current';

is to fill existing table rows with 'old', not 'current'.  That's
intentional and desirable for a couple of reasons:

* It makes the behavior the same whether you merge the sub-commands
into one ALTER command or give them separately;

* If we applied the new default while filling the table, there would
be no way to get the existing behavior in one SQL command.

The same reasoning applies in cases that add a column and then
manipulate its GENERATED/IDENTITY status in a second sub-command,
since the generation expression is really just a kind of default.
However, that wasn't very obvious (at least not to me; earlier in
the referenced discussion thread I'd thought it was a bug to be
fixed).  And it certainly wasn't documented.

Hence, add documentation, code comments, and a test case to clarify
that this behavior is all intentional.

In passing, adjust ATExecAddColumn's defaults-related relkind check
so that it matches up exactly with ATRewriteTables, instead of being
effectively (though not literally) the negated inverse condition.
The reasoning can be explained a lot more concisely that way, too
(not to mention that the comment now matches the code, which it
did not before).

Discussion: https://postgr.es/m/10365.1558909428@sss.pgh.pa.us
This commit is contained in:
Tom Lane
2020-01-21 16:17:21 -05:00
parent affdde2e15
commit 9b9c5f279e
4 changed files with 58 additions and 11 deletions
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18
src/backend/commands/tablecmds.c
View File

@ -6126,14 +6126,18 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
* returned by AddRelationNewConstraints, so that the right thing happens
* when a datatype's default applies.
*
* We skip this step completely for views and foreign tables. For a view,
* we can only get here from CREATE OR REPLACE VIEW, which historically
* doesn't set up defaults, not even for domain-typed columns. And in any
* case we mustn't invoke Phase 3 on a view or foreign table, since they
* have no storage.
* Note: it might seem that this should happen at the end of Phase 2, so
* that the effects of subsequent subcommands can be taken into account.
* It's intentional that we do it now, though. The new column should be
* filled according to what is said in the ADD COLUMN subcommand, so that
* the effects are the same as if this subcommand had been run by itself
* and the later subcommands had been issued in new ALTER TABLE commands.
*
* We can skip this entirely for relations without storage, since Phase 3
* is certainly not going to touch them. System attributes don't have
* interesting defaults, either.
*/
if (relkind != RELKIND_VIEW && relkind != RELKIND_COMPOSITE_TYPE
&& relkind != RELKIND_FOREIGN_TABLE && attribute.attnum > 0)
if (RELKIND_HAS_STORAGE(relkind) && attribute.attnum > 0)
{
/*
* For an identity column, we can't use build_column_default(),