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

On Tue, Feb 2, 2016 at 3:59 AM, Thom Brown <thom@linux.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.oidfrom (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