Re: WIP: Covering + unique indexes.

On Tue, Apr 10, 2018 at 5:45 PM, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> I did find another problem, though. Looks like the idea to explicitly
> represent the number of attributes directly has paid off already:
>
> pg(at)~[3711]=# create table covering_bug (f1 int, f2 int, f3 text);
> create unique index cov_idx on covering_bug (f1) include(f2);
> insert into covering_bug select i, i * random() * 1000, i * random() *
> 100000 from generate_series(0,100000) i;
> DEBUG: building index "pg_toast_16451_index" on table "pg_toast_16451" serially
> CREATE TABLE
> DEBUG: building index "cov_idx" on table "covering_bug" serially
> CREATE INDEX
> ERROR: tuple has wrong number of attributes in index "cov_idx"

Actually, this was an error on my part (though I'd still maintain that
the check paid off here!). I'll still add defensive assertions inside
_bt_newroot(), and anywhere else that they're needed. There is no
reason to not add defensive assertions in all code that handles page
splits, and needs to fetch a highkey from some other page. We missed a
few of those.

I'll add an item to "Decisions to Recheck Mid-Beta" section of the
open items page for this patch. We should review the decision to make
a call to _bt_check_natts() within _bt_compare(). It might work just
as well as an assertion, and it would be unfortunate if workloads that
don't use covering indexes had to pay a price for the
_bt_check_natts() call, even if it was a small price. I've seen
_bt_compare() appear prominently in profiles quite a few times.

--
Peter Geoghegan