Re: xml data type implications of no =

On 25/05/10 16:43, Mark Kirkwood wrote:
> Today I ran into some interesting consequences of the xml data type
> being without an "=" operator. One I thought I'd post here because it
> has a *possible* planner impact. I'm not sure it is actually a bug as
> such, but this seemed the best forum to post in initially:
>
> test=# \d bug
> Table "public.bug"
> Column | Type | Modifiers
> --------+---------+-----------
> id | integer |
> val | xml |
>
> test=# explain select val::text from bug;
> QUERY PLAN
> --------------------------------------------------------------
> Seq Scan on bug (cost=0.00..58127.78 rows=1000278 width=32)
>
>
> Note the width estimate. However a more realistic estimate for width is:
>
> test=# select 8192/(reltuples/relpages) as width from pg_class where
> relname='bug';
> width
> ------------------
> 394.130431739976
>
> So we are going to massively underestimate the "size" of such a
> dataset. Now this appears to be a consequence of no "=" operator
> (std_typanalyze in analyze.c bails if there isn't one), so the planner
> has no idea about how wide 'val' actually is. I'm wondering if it is
> worth having at least an "=" operator to enable some minimal stats to
> be available for xml columns.
>

Adding a minimal = op (see attached) and an analyze results in:

test=# explain select val::text from bug;
QUERY PLAN
---------------------------------------------------------------
Seq Scan on bug (cost=0.00..62632.08 rows=1000008 width=385)

which gives a much better indication of dataset size.