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 <ogauthe...@gautherot.net> 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 regards > Olivier > > > On Mon, Dec 16, 2019, 07:00 Shijia Wei <shijia...@utexas.edu> 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.sql >> explain (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 shijia...@utexas.edu | https://0x161e-swei.github.io
