Hi Olivier,
I do not think that the queries are executed concurrently. The bash for loop ensures that the next command fires only after the first returns.
Also for some 'complex' queries, even a wait-period that is longer than the total execution time does not completely avoid this effect.
For example, a wait-period of 5-second in between queries that take 2-second to run, does not help avoid the increasing runtime problem completely.
Thanks,
Shijia
On Mon, Dec 16, 2019 at 2:04 AM Olivier Gautherot <ogautherot@xxxxxxxxxxxxx> wrote:
Hi Shijia,
It sounds like concurrency on the queries: the second starts before the first ends, and so on. With a short wait in between you ensure sequential execution. Notice that you also have the overhead of concurrent psql...Sounds normal to me.Best regardsOlivierOn Mon, Dec 16, 2019, 07:00 Shijia Wei <shijiawei@xxxxxxxxxx> wrote:Hi all,I am running TPC-H on recent postgresql (12.0 and 12.1).On some of the queries (that may involve parallel scans) I see this interesting behavior:When these queries are executed back-to-back (sent from psql interactive terminal), the total execution time of them increase monotonically.I simplified query-1 to demonstrate this effect:``` example.sqlexplain (analyze, buffers) select
max(l_shipdate) as max_data,
count(*) as count_order
from
lineitem
where
l_shipdate <= date '1998-12-01' - interval '20' day;
```When I execute (from fish) following command:
`for i in (seq 1 20); psql tpch < example.sql | grep Execution; end`
The results I get are as follows:
"
Execution Time: 184.864 ms
Execution Time: 192.758 ms
Execution Time: 197.380 ms
Execution Time: 200.384 ms
Execution Time: 202.950 ms
Execution Time: 205.695 ms
Execution Time: 208.082 ms
Execution Time: 209.108 ms
Execution Time: 212.428 ms
Execution Time: 214.539 ms
Execution Time: 215.799 ms
Execution Time: 219.057 ms
Execution Time: 222.102 ms
Execution Time: 223.779 ms
Execution Time: 227.819 ms
Execution Time: 229.710 ms
Execution Time: 239.439 ms
Execution Time: 237.649 ms
Execution Time: 249.178 ms
Execution Time: 261.268 ms
"In addition, if the repeated more times, the total execution time can end up being 10X and more!!!When there a wait period in-between queries, (e.g. sleep 10) in the above for loop, this increasing execution time behavior goes a way.For more complex queries, the "wait period" needs to be longer to avoid the increase in execution time.
Some metadata about this table "lineitem":tpch=# \d lineitem
Table "public.lineitem"
Column | Type | Collation | Nullable | Default
-----------------+-----------------------+-----------+----------+---------
l_orderkey | integer | | not null |
l_partkey | integer | | not null |
l_suppkey | integer | | not null |
l_linenumber | integer | | not null |
l_quantity | numeric(15,2) | | not null |
l_extendedprice | numeric(15,2) | | not null |
l_discount | numeric(15,2) | | not null |
l_tax | numeric(15,2) | | not null |
l_returnflag | character(1) | | not null |
l_linestatus | character(1) | | not null |
l_shipdate | date | | not null |
l_commitdate | date | | not null |
l_receiptdate | date | | not null |
l_shipinstruct | character(25) | | not null |
l_shipmode | character(10) | | not null |
l_comment | character varying(44) | | not null |
Indexes:
"i_l_commitdate" btree (l_commitdate)
"i_l_orderkey" btree (l_orderkey)
"i_l_orderkey_quantity" btree (l_orderkey, l_quantity)
"i_l_partkey" btree (l_partkey)
"i_l_receiptdate" btree (l_receiptdate)
"i_l_shipdate" btree (l_shipdate)
"i_l_suppkey" btree (l_suppkey)
"i_l_suppkey_partkey" btree (l_partkey, l_suppkey)tpch=# SELECT relname, relpages, reltuples, relallvisible, relkind, relnatts, relhassubclass, reloptions, pg_table_size(oid) FROM pg_class WHERE relname='lineitem';
relname | relpages | reltuples | relallvisible | relkind | relnatts | relhassubclass | reloptions | pg_table_size
----------+----------+--------------+---------------+---------+----------+----------------+------------+---------------
lineitem | 112503 | 6.001167e+06 | 112503 | r | 16 | f | | 921903104
(1 row)Postgresql 12.0 and 12.1 are all manually installed from source.Both are running on Ubuntu 16.04 kernel 4.4.0-142-generic, on Intel(R) Core(TM) i7-6700K.Any help greatly appreciated!Shijia
Shijia Wei
ECE, UT Austin | ACSES | 3rd Year PhD
ECE, UT Austin | ACSES | 3rd Year PhD
