Thread: incoherent dead tuples between pg_stat_user_tables and pgstattuple?
Hello, running 11.5 I've some misconception about pgstatuple: it seems it lags behind normal statistics. I've almost double a table with an update. The result is: testdb=# select n_live_tup, n_dead_tup, last_autovacuum, last_autoanalyze from pg_stat_user_tables where relname = 'foo'; -[ RECORD 1 ]----+------------------------------ n_live_tup | 5021619 n_dead_tup | 5000000 last_autovacuum | 2019-09-27 12:05:36.072376+02 last_autoanalyze | 2019-09-27 12:06:16.538413+02 but pgstatuple provides a quite different output, saying the table is not full of dead rows: testdb=# SELECT * FROM pgstattuple( 'foo' ); -[ RECORD 1 ]------+---------- table_len | 930897920 tuple_count | 5000000 tuple_len | 439595972 tuple_percent | 47.22 dead_tuple_count | 88 dead_tuple_len | 7744 dead_tuple_percent | 0 free_space | 447716392 free_percent | 48.1 Even after running an ANALYZE, pgstattuple provides the same results. After a VACUUM FULL ANALYZE the world is as I would expect it to be: testdb=# select n_live_tup, n_dead_tup, last_autovacuum, last_autoanalyze from pg_stat_user_tables where relname = 'foo'; -[ RECORD 1 ]----+------------------------------ n_live_tup | 5000117 n_dead_tup | 0 last_autovacuum | 2019-09-27 12:05:36.072376+02 last_autoanalyze | 2019-09-27 12:06:16.538413+02 Time: 16,159 ms testdb=# SELECT * FROM pgstattuple( 'foo' ); -[ RECORD 1 ]------+---------- table_len | 465453056 tuple_count | 5000000 tuple_len | 439595972 tuple_percent | 94.44 dead_tuple_count | 0 dead_tuple_len | 0 dead_tuple_percent | 0 free_space | 3870144 free_percent | 0.83 Am I missing something? Thanks, Luca
Luca Ferrari <fluca1978@gmail.com> writes:
> running 11.5 I've some misconception about pgstatuple: it seems it
> lags behind normal statistics.
n_live_tup/n_dead_tup should not be thought to be better than
approximations. Most operations adjust them only incrementally,
and messages to the stats collector can get dropped entirely
under heavy load, causing deltas to go missing. There are
also race conditions involved in some update scenarios.
> Even after running an ANALYZE, pgstattuple provides the same results.
> After a VACUUM FULL ANALYZE the world is as I would expect it to be:
VACUUM FULL is one of very few operations that reset those counters
to guaranteed-correct values (and I'm not sure the guarantee is
ironclad even in that case).
It's very hard to improve on this without giving up the desirable
ability to have concurrent table updates. If you really want an
accurate row count, COUNT(*) or pgstattuples will give you a more
reliable estimate ... at much higher cost, of course.
regards, tom lane
On Fri, Sep 27, 2019 at 3:52 PM Tom Lane <tgl@sss.pgh.pa.us> wrote: > n_live_tup/n_dead_tup should not be thought to be better than > approximations. Most operations adjust them only incrementally, > and messages to the stats collector can get dropped entirely > under heavy load, causing deltas to go missing. There are > also race conditions involved in some update scenarios. > Are you suggesting tha autovacuum made free space available so that pg_stat_user_tables lags behind? Because I don't see last_autovacuum changing in the mantime. Luca
Luca Ferrari <fluca1978@gmail.com> writes:
> On Fri, Sep 27, 2019 at 3:52 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> n_live_tup/n_dead_tup should not be thought to be better than
>> approximations. Most operations adjust them only incrementally,
>> and messages to the stats collector can get dropped entirely
>> under heavy load, causing deltas to go missing. There are
>> also race conditions involved in some update scenarios.
> Are you suggesting tha autovacuum made free space available so that
> pg_stat_user_tables lags behind? Because I don't see last_autovacuum
> changing in the mantime.
I'm not suggesting any specific scenario, because you haven't presented
any evidence as to when those counts became off. I'm just saying that
there are plenty of possible mechanisms for them to become so.
regards, tom lane
On Fri, Sep 27, 2019 at 4:43 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> I'm not suggesting any specific scenario, because you haven't presented
> any evidence as to when those counts became off. I'm just saying that
> there are plenty of possible mechanisms for them to become so.
So far autovacuum seems the main reason for pg_stattuple to be _ahead_
of pg_stats (I stand correct).
In fact, turning off autovacuum the pg_stattuple shows the exact
result as I would have expected, and probably the pg_stats I was
referring to with n_dead_tup > 0 was due to a previous UPDATE ran
against the same table.
Apparently this produces always the same results:
testdb=# CREATE TABLE foo( bar text );
CREATE TABLE
testdb=# INSERT INTO foo
SELECT generate_series( 1, 5000000 )::text;
INSERT 0 5000000
testdb=# ANALYZE foo;
ANALYZE
testdb=# show autovacuum;
-[ RECORD 1 ]---
autovacuum | off
testdb=# select current_timestamp;
-[ RECORD 1 ]-----+------------------------------
current_timestamp | 2019-09-27 20:44:39.277217+02
Time: 0,839 ms
testdb=# select n_live_tup, n_dead_tup, last_autovacuum,
last_autoanalyze from pg_stat_user_tables where relname = 'foo';
-[ RECORD 1 ]----+------------------------------
n_live_tup | 5000000
n_dead_tup | 0
last_autovacuum | 2019-09-27 20:29:18.571858+02
last_autoanalyze | 2019-09-27 20:29:40.889657+02
testdb=# UPDATE foo SET bar = lower( bar );
UPDATE 5000000
testdb=# select n_live_tup, n_dead_tup, last_autovacuum,
last_autoanalyze from pg_stat_user_tables where relname = 'foo';
-[ RECORD 1 ]----+------------------------------
n_live_tup | 5000000
n_dead_tup | 5000000
last_autovacuum | 2019-09-27 20:29:18.571858+02
last_autoanalyze | 2019-09-27 20:29:40.889657+02
testdb=# SELECT * FROM pgstattuple( 'foo' );
-[ RECORD 1 ]------+----------
table_len | 362479616
tuple_count | 5000000
tuple_len | 158888896
tuple_percent | 43.83
dead_tuple_count | 5000000
dead_tuple_len | 158888896
dead_tuple_percent | 43.83
free_space | 1240672
free_percent | 0.34