On Tue, Aug 9, 2016 at 4:50 AM, Vladimir Sitnikov
<sitnikov.vladimir@gmail.com> wrote:
> I've tried pgbench -M prepared, and it is way faster than pgbench -M simple.
That's true, but it's also testing something completely different from
what Shay is concerned about. -M prepared creates a prepared
statement once, and then executes it many times. That is, as you say,
faster. But what Shay is concerned about is the case where you are
using the extended query protocol to send out-of-line parameters but
for some reason you can't use prepared statements, for example because
the queries are dynamically generated and you have to keep sending a
different query text every time. That case is analogous to -M
extended, not -M prepared. And -M extended is well-known to be SLOWER
than -M simple. Here's a quick test on my laptop demonstrating this:
[rhaas ~]$ pgbench -M simple -S -T 10
starting vacuum...end.
transaction type: <builtin: select only>
scaling factor: 1
query mode: simple
number of clients: 1
number of threads: 1
duration: 10 s
number of transactions actually processed: 119244
latency average: 0.084 ms
tps = 11919.440321 (including connections establishing)
tps = 11923.229139 (excluding connections establishing)
[rhaas ~]$ pgbench -M prepared -S -T 10
starting vacuum...end.
transaction type: <builtin: select only>
scaling factor: 1
query mode: prepared
number of clients: 1
number of threads: 1
duration: 10 s
number of transactions actually processed: 192100
latency average: 0.052 ms
tps = 19210.341944 (including connections establishing)
tps = 19214.820999 (excluding connections establishing)
[rhaas ~]$ pgbench -M extended -S -T 10
starting vacuum...end.
transaction type: <builtin: select only>
scaling factor: 1
query mode: extended
number of clients: 1
number of threads: 1
duration: 10 s
number of transactions actually processed: 104275
latency average: 0.096 ms
tps = 10425.510813 (including connections establishing)
tps = 10427.725239 (excluding connections establishing)
Shay is not worried about the people who are getting 19.2k TPS instead
of 11.9k TPS. Those people are already happy. He is worried about
the people who are getting 10.4k TPS instead of 11.9k TPS. People who
can't use prepared statements because their query text varies - and I
have personally written multiple web applications that have exactly
that profile - suffer a big penalty if they choose to use the extended
query protocol to pass parameters. Here, it's about a 13% performance
loss.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company