How to Find Tables Accessed During Last Period in PostgreSQL

access-controlpostgresqltimestamptrigger

I want to check which tables have been updated during a given period, for instance in descending order of access time per table.

How can I get that for PostgreSQL?

Best Answer

You can get some information about the last change to a table with xmin, eg:

select max(xmin::text::bigint) from t;

But you need to be aware of many caveats including modulo and wraparound and frozen xids.

testbed:

set role dba;
create role stack;
grant stack to dba;
create schema authorization stack;
set role stack;
--
create or replace function f(p_schema in text, p_table in text) 
                  returns integer language plpgsql immutable as $$
declare
  n integer;
begin
  execute 'select max(xmin::text::bigint) from '||p_schema||'.'||p_table into n;
  return n;
end;$$;
--
create table foo as select generate_series(1, 100) as id;
create table bar as select generate_series(1, 100) as id;
create table baz as select generate_series(1, 100) as id;
--

method:

select table_name, f(table_schema, table_name)
from information_schema.tables
where table_schema='stack'
order by 2 desc;
/*
 table_name |   f
------------+--------
 baz        | 784657
 bar        | 784656
 foo        | 784655
*/
--
update foo set id=id+1 where id=100;
--
select table_name, f(table_schema, table_name)
from information_schema.tables
where table_schema='stack'
order by 2 desc;
/*
 table_name |   f
------------+--------
 foo        | 784658
 baz        | 784657
 bar        | 784656
*/

cleanup:

drop schema stack cascade;
set role dba;
drop role stack;
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