On Thu, Sep 12, 2019 at 11:08:28AM -0400, Tom Lane wrote:
Arseny Sher <a.s...@postgrespro.ru> writes:
A problem of similar nature can be reproduced with the following
stripped-down scenario:
CREATE TABLE pears(f1 int primary key, f2 int);
INSERT INTO pears SELECT i, i+1 FROM generate_series(1, 100) i;
CREATE OR REPLACE FUNCTION pears_f(i int) RETURNS int LANGUAGE SQL IMMUTABLE AS
$$
SELECT f1 FROM pears WHERE pears.f2 = 42
$$;
CREATE index ON pears ((pears_f(f1)));
We've seen complaints about this sort of thing before, and rejected
them because, as you say, that function is NOT immutable. When you
lie to the system like that, you should not be surprised if things
break.
There is already a mechanism which prevents usage of indexes during
reindex -- ReindexIsProcessingIndex et al. However, to the contrary of
what index.c:3664 comment say, these protect only indexes on system
catalogs, not user tables: the only real caller is genam.c.
Attached patch extends it: the same check is added to
get_relation_info. Also SetReindexProcessing is cocked in index_create
to defend from index self usage during creation as in stripped example
above. There are some other still unprotected callers of index_build;
concurrent index creation doesn't need it because index is
'not indisvalid' during the build, and in RelationTruncateIndexes
table is empty, so it looks like it can be omitted.
I have exactly no faith that this fixes things enough to make such
cases supportable. And I have no interest in opening that can of
worms anyway. I'd rather put in some code to reject database
accesses in immutable functions.
Same here. My hunch is a non-trivaial fraction of applications using
this "trick" is silently broken in various subtle ways.
One might argue that function selecting from table can hardly be called
immutable, and immutability is required for index expressions. However,
if user is sure table contents doesn't change, why not?
If the table contents never change, why are you doing VACUUM FULL on it?
It's possible the columns referenced by the index expression are not
changing, but some additional columns are updated.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
