With PL/pgSQL:
create type foo as (a int, b int);
create function bar() returns record as $$
declare
r foo := row(123, 2^30);
begin
alter type foo alter attribute b type text;
return r;
end;
$$ language plpgsql;
postgres=# select bar();
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
LOG: server process (PID 394076) was terminated by signal 11: Segmentation
fault
(Here 2^30 is interpreted as a string length.)
ISTM after the alter operation, the attribute description of 'b' has
been altered to cope with type text, but the data layout inside the heap
tuple stays unchanged. So when we fetch attribute 'b', what we get is
an integer pointer into the tuple's data area storing value 2^30, due to
type text is not attbyval.
Then later we interpret that integer pointer as a varlena pointer, which
is not correct.
But I'm not sure how to fix it. Is there an easy way to also alter the
data layout inside the tuple?
From the docs:
The variants to add and drop attributes are part of the SQL standard; the other variants are PostgreSQL extensions.
I think I understand why the standard didn't include "ALTER TYPE ... SET DATA TYPE"...
We didn't even add the USING clause that exists for ALTER TABLE
(realizes there is no table involved, and that MVCC prevents this particular pl/pgsql scoped issue generally)
We don't need to fix this though (i.e., make the query continue to somehow work), we need to detect the situation and return some kind of error as opposed to crashing the server. The transaction itself is doing something illegal and can be forced to abort. Alternatively, maybe the ALTER TYPE command can emit the error that it is unable to alter the type as it is already in use by the current session. But that would close off possible non-problematic uses of ALTER TYPE in this situation
I don't see how you could generalize altering the data layout inside the tuple without a USING clause stating how to cast the stored value to the new type - the USING clause exists for a reason in ALTER TABLE.
Apparently there is a cache invalidation that happens at CCI on the ALTER TYPE and when the new type definition shows up extra and missing columns are added (null) or removed as needed (i.e., neither ADD nor DROP cause issues). Which suggests that the desired solution is one that simply converts the SEGFAULT into a non-server-crashing error and document the general inability (or desire, IMO) to do better in a back-patchable way (even if the user isn't getting a segfault there is still a problem - just replace "text" in the example with "bigint".
David J.