Wednesday, January 28 2009
PostgreSQL, Catalogs

Controling HOT usage in 8.3

As it happens, I've got some environments where I want to make sure HOT ( aka Heap Only Tuples) is in use. Because we're doing so much updates a second that I want to get sure it's not killing my database server. I not only wrote some checking view to see about it, but also made a quick article about it in the French PostgreSQL website. Handling around in #postgresql means that I'm now bound to write about it in English too!

So HOT will get used each time you update a row without changing an indexed value of it, and the benefit is skipping index maintenance, and as far as I understand it, easying vacuum hard work too. To get the benefit, HOT will need some place where to put new version of the UPDATEd tuple in the same disk page, which means you'll probably want to set your table fillfactor to something much less than 100.

Now, here's how to check you're benefitting from HOT:

SELECT schemaname, relname,
       n_tup_upd,n_tup_hot_upd,
       case when n_tup_upd > 0
            then ((n_tup_hot_upd::numeric/n_tup_upd::numeric)*100.0)::numeric(5,2) 
            else NULL
       end AS hot_ratio
 
 FROM pg_stat_all_tables;
 
 schemaname | relname | n_tup_upd | n_tup_hot_upd | hot_ratio
------------+---------+-----------+---------------+-----------
 public     | table1  |         6 |             6 |    100.00
 public     | table2  |   2551200 |       2549474 |     99.93

Here's even an extended version of the same request, displaying the fillfactor option value for the tables you're inquiring about. This comes separated from the first example because you get the fillfactor of a relation into the pg_class catalog reloptions field, and to filter against a schema qualified table name, you want to join against pg_namespace too.

SELECT t.schemaname, t.relname, c.reloptions, 
       t.n_tup_upd, t.n_tup_hot_upd, 
       case when n_tup_upd > 0 
            then ((n_tup_hot_upd::numeric/n_tup_upd::numeric)*100.0)::numeric(5,2)
            else NULL
        end AS hot_ratio
FROM pg_stat_all_tables t 
      JOIN (pg_class c JOIN pg_namespace n ON c.relnamespace = n.oid) 
        ON n.nspname = t.schemaname AND c.relname = t.relname
 
 schemaname | relname |   reloptions    | n_tup_upd | n_tup_hot_upd | hot_ratio
------------+---------+-----------------+-----------+---------------+-----------
 public     | table1  | {fillfactor=50} |   1585920 |       1585246 |     99.96
 public     | table2  | {fillfactor=50} |   2504880 |       2503154 |     99.93

Don't let the HOT question affect your sleeping no more!