On 05/17/2014 11:12 PM, Tom Lane wrote:
I wrote:
BTW, the *real* problem with all this stuff is that the gbtreekeyNN types
are declared as having int alignment, even though some of the opclasses
store double-aligned types in them.  I imagine it's possible to provoke
bus errors on machines that are picky about alignment.  The first column
of an index is safe enough because index tuples will be double-aligned
anyway, but it seems like there's a hazard for lower-order columns.


And indeed there is:

regression=# create extension btree_gist ;
CREATE EXTENSION
regression=# create table tt (f1 int2, f2 float8);
CREATE TABLE
regression=# create index on tt using gist(f1,f2);
CREATE INDEX
regression=# insert into tt values(1,2);
INSERT 0 1
regression=# insert into tt values(2,4);
INSERT 0 1
regression=# set enable_seqscan TO 0;
SET
regression=# explain select * from tt where f1=2::int2 and f2=4;
                                QUERY PLAN
------------------------------------------------------------------------
  Index Scan using tt_f1_f2_idx on tt  (cost=0.14..8.16 rows=1 width=10)
    Index Cond: ((f1 = 2::smallint) AND (f2 = 4::double precision))
  Planning time: 1.043 ms
(3 rows)

regression=# select * from tt where f1=2::int2 and f2=4;
<< bus error >>

This is something we cannot fix compatibly :-(

AFAICS, what we have to do is mark the wider gbtreekeyNN types as
requiring double alignment.  This will break pg_upgrade'ing any index in
which they're used as non-first columns, unless perhaps all the preceding
columns have at least double size/alignment.  I guess pg_upgrade can
check for that, but it'll be kind of a pain.

Another issue is what the heck btree_gist's extension upgrade script ought
to do about this.  It can't just go and modify the type declarations.

Actually, on further thought, this isn't an issue for pg_upgrade at all,
just for the extension upgrade script.  Maybe we just have to make it
poke through the catalogs looking for at-risk indexes, and refuse to
complete the extension upgrade if there are any?

I think it would be best to just not allow pg_upgrade if there are any indexes using the ill-defined types. The upgrade script could then simply drop the types and re-create them. The DBA would need to drop the affected indexes before upgrade and re-create them afterwards, but I think that would be acceptable. I doubt there are many people using btree_gist on int8 or float8 columns.

Another way to attack this would be to change the code to memcpy() the values before accessing them. That would be ugly, but it would be back-patchable. In HEAD, I'd rather bite the bullet and get the catalogs fixed, though.

- Heikki


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