zero value in statistics collector's result

Hi,

When I use the statistics collector to see the number of IO, I always get
zero in almost all of columns. I really want to know the reason for that.

The result of statistics view:

# select * from pg_statio_user_tables;

relid | schemaname | relname | heap_blks_read | heap_blks_hit |
idx_blks_read | idx_blks_hit | toast_blks_read | toast_blks_hit |
tidx_blks_read | tidx_blks_hit

-------+------------+---------+----------------+---------------+------------
---+--------------+-----------------+----------------+----------------+-----
----------

16386 | public | tab | 0 | 0 |
0 | 0 | | | |

(1 row)

# select * from pg_statio_user_indexes;

relid | indexrelid | schemaname | relname | indexrelname | idx_blks_read |
idx_blks_hit

-------+------------+------------+---------+--------------+---------------+-
-------------

16386 | 24581 | public | tab | idx_tab_a2 | 0 |
0

(1 row)

I've set:

stats_start_collector = on

stats_row_level = on

stats_block_level = on

and I think that the statistics collector should be running, because:

$ ps aux|grep stats

postgres 3688 0.0 0.0 7272 648 ? Ss 00:48 0:00 postgres:
stats collector process

postgres 29790 0.0 0.0 4004 712 pts/2 S+ 04:44 0:00 grep stats

Any help would be really appreciated.

Regards,

Yinan