Re: [WIP] Effective storage of duplicates in B-tree index.

On Tue, Feb 2, 2016 at 3:59 AM, Thom Brown <thom(at)linux(dot)com> wrote:
> public | pgbench_accounts_pkey | index | thom | pgbench_accounts | 214 MB |
> public | pgbench_branches_pkey | index | thom | pgbench_branches | 24 kB |
> public | pgbench_tellers_pkey | index | thom | pgbench_tellers | 48 kB |

I see the same.

I use my regular SQL query to see the breakdown of leaf/internal/root pages:

postgres=# with tots as (
SELECT count(*) c,
avg(live_items) avg_live_items,
avg(dead_items) avg_dead_items,
u.type,
r.oid
from (select c.oid,
c.relpages,
generate_series(1, c.relpages - 1) i
from pg_index i
join pg_opclass op on i.indclass[0] = op.oid
join pg_am am on op.opcmethod = am.oid
join pg_class c on i.indexrelid = c.oid
where am.amname = 'btree') r,
lateral (select * from bt_page_stats(r.oid::regclass::text, i)) u
group by r.oid, type)
select ct.relname table_name,
tots.oid::regclass::text index_name,
(select relpages - 1 from pg_class c where c.oid = tots.oid) non_meta_pages,
upper(type) page_type,
c npages,
to_char(avg_live_items, '990.999'),
to_char(avg_dead_items, '990.999'),
to_char(c/sum(c) over(partition by tots.oid) * 100, '990.999') || '
%' as prop_of_index
from tots
join pg_index i on i.indexrelid = tots.oid
join pg_class ct on ct.oid = i.indrelid
where tots.oid = 'pgbench_accounts_pkey'::regclass
order by ct.relnamespace, table_name, index_name, npages, type;
table_name │ index_name │ non_meta_pages │ page_type
│ npages │ to_char │ to_char │ prop_of_index
──────────────────┼───────────────────────┼────────────────┼───────────┼────────┼──────────┼──────────┼───────────────
pgbench_accounts │ pgbench_accounts_pkey │ 27,421 │ R
│ 1 │ 97.000 │ 0.000 │ 0.004 %
pgbench_accounts │ pgbench_accounts_pkey │ 27,421 │ I
│ 97 │ 282.670 │ 0.000 │ 0.354 %
pgbench_accounts │ pgbench_accounts_pkey │ 27,421 │ L
│ 27,323 │ 366.992 │ 0.000 │ 99.643 %
(3 rows)

But this looks healthy -- I see the same with master. And since the
accounts table is listed as 1281 MB, this looks like a plausible ratio
in the size of the table to its primary index (which I would not say
is true of an 87MB primary key index).

Are you sure you have the details right, Thom?
--
Peter Geoghegan