database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-07-28 23:42:10 +03:00
Code Activity

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

Browse Source 
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
Download Patch File Download Diff File Expand all files Collapse all files

39
doc/src/sgml/ref/alter_table.sgml
View File

@ -203,10 +203,11 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
<term><literal>SET</literal>/<literal>DROP DEFAULT</literal></term>
<listitem>
<para>
These forms set or remove the default value for a column.
Default values only apply in subsequent <command>INSERT</command>
or <command>UPDATE</command> commands; they do not cause rows already in the
table to change.
These forms set or remove the default value for a column (where
removal is equivalent to setting the default value to NULL). The new
default value will only apply in subsequent <command>INSERT</command>
or <command>UPDATE</command> commands; it does not cause rows already
in the table to change.
</para>
</listitem>
</varlistentry>
@ -268,6 +269,10 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
These forms change whether a column is an identity column or change the
generation attribute of an existing identity column.
See <xref linkend="sql-createtable"/> for details.
Like <literal>SET DEFAULT</literal>, these forms only affect the
behavior of subsequent <command>INSERT</command>
and <command>UPDATE</command> commands; they do not cause rows
already in the table to change.
</para>
<para>
@ -1370,6 +1375,32 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
<programlisting>
ALTER TABLE distributors ADD COLUMN address varchar(30);
</programlisting>
That will cause all existing rows in the table to be filled with null
values for the new column.
</para>
<para>
To add a column with a non-null default:
<programlisting>
ALTER TABLE measurements
ADD COLUMN mtime timestamp with time zone DEFAULT now();
</programlisting>
Existing rows will be filled with the current time as the value of the
new column, and then new rows will receive the time of their insertion.
</para>
<para>
To add a column and fill it with a value different from the default to
be used later:
<programlisting>
ALTER TABLE transactions
ADD COLUMN status varchar(30) DEFAULT 'old',
ALTER COLUMN status SET default 'current';
</programlisting>
Existing rows will be filled with <literal>old</literal>, but then
the default for subsequent commands will be <literal>current</literal>.
The effects are the same as if the two sub-commands had been issued
in separate <command>ALTER TABLE</command> commands.
</para>
<para>