Re: Find out the indexed columns

Nikhil Ingale <niks(dot)bgm(at)gmail(dot)com> writes:
> I have a condition based index created. I see that the postgres metadata
> table doesn't return columns associated with condition based indexes.

No, it gives you back the indexed expression. Showing only the
columns involved would be very misleading.

> How do I find out the associated columns for such an index?

Probably the most future-proof way is to look into pg_depend to see
what columns the index depends on. For example:

regression=# create table t (f1 float8);
CREATE TABLE
regression=# create index ti on t (sin(f1));
CREATE INDEX
regression=# select * from pg_depend where classid = 'pg_class'::regclass and objid = 'ti'::regclass;
classid | objid | objsubid | refclassid | refobjid | refobjsubid | deptype
---------+-------+----------+------------+----------+-------------+---------
1259 | 48626 | 0 | 1259 | 48623 | 0 | a
1259 | 48626 | 0 | 1259 | 48623 | 1 | a
(2 rows)

What we have here is one dependency on the whole table t (the row
with refobjsubid = 0) and one on just the used column (refobjsubid = 1).
(There could be dependencies on things other than tables and columns,
for example any user-defined functions in the expression would have
entries too.)

You could get back the column name(s) by joining to pg_attribute,
along the lines of

regression=# select attname from pg_depend join pg_attribute a on (refobjid = a.attrelid and refobjsubid = a.attnum) where classid = 'pg_class'::regclass and objid = 'ti'::regclass and refclassid = 'pg_class'::regclass;
attname
---------
f1
(1 row)

Don't skip the classid/refclassid constraints. It might seem to work
without those, but someday you'll hit duplicate-across-catalogs OIDs
and get wrong answers. On the other hand, I've relied on the join to
get rid of the refobjsubid = 0 entry; you could consider filtering
that explicitly as well.

regards, tom lane