Thread

1. impact of auto explain on overall performance

   Stephan Schmidt <schmidt@dltmail.de> — 2019-03-14T07:29:17Z

   Hello,
   
   i’m currently working on a high Performance Database and want to make sure that whenever there are slow queries during regular operations i’ve got all Information about the query in my logs. So auto_explain come to mind, but the documentation explicitly states that it Comes at a cost. My Question is, how big is the latency added by auto_explain in percentage or ms ?
   
   Best regards,
   
   stephan
   

2. Re: impact of auto explain on overall performance

   Justin Pryzby <pryzby@telsasoft.com> — 2019-03-14T08:23:00Z

   On Thu, Mar 14, 2019 at 07:29:17AM +0000, Stephan Schmidt wrote:
   > i’m currently working on a high Performance Database and want to make sure that whenever there are slow queries during regular operations i’ve got all Information about the query in my logs. So auto_explain come to mind, but the documentation explicitly states that it Comes at a cost. My Question is, how big is the latency added by auto_explain in percentage or ms ?
   
   https://www.postgresql.org/docs/current/auto-explain.html
   |log_analyze
   ...
   |When this parameter is on, per-plan-node timing occurs for all statements executed, whether or not they run long enough to actually get logged. This can have an extremely negative impact on performance. Turning off auto_explain.log_timing ameliorates the performance cost, at the price of obtaining less information.
   
   |auto_explain.log_timing (boolean)
   |auto_explain.log_timing controls whether per-node timing information is printed when an execution plan is logged; it's equivalent to the TIMING option of EXPLAIN. The overhead of repeatedly reading the system clock can slow down queries significantly on some systems, so it may be useful to set this parameter to off when only actual row counts, and not exact times, are needed. This parameter has no effect unless auto_explain.log_analyze is enabled. This parameter is on by default. Only superusers can change this setting.
   
   I believe the cost actually varies significantly with the type of plan "node",
   with "nested loops" incurring much higher overhead.
   
   I think you could compare using explain(analyze) vs explain(analyze,timing
   off).  While you're at it, compare without explain at all.
   
   I suspect the overhead is inconsequential if you set log_timing=off and set
   log_min_duration such that only the slowest queries are logged.
   
   Then, you can manually run "explain (analyze,costs on)" on any problematic
   queries to avoid interfering with production clients.
   
   Justin
   
   
   

3. Re: impact of auto explain on overall performance

   Adrien Nayrat <adrien.nayrat@anayrat.info> — 2019-03-14T09:08:39Z

   On 3/14/19 9:23 AM, Justin Pryzby wrote:
   > On Thu, Mar 14, 2019 at 07:29:17AM +0000, Stephan Schmidt wrote:
   >> i’m currently working on a high Performance Database and want to make sure that whenever there are slow queries during regular operations i’ve got all Information about the query in my logs. So auto_explain come to mind, but the documentation explicitly states that it Comes at a cost. My Question is, how big is the latency added by auto_explain in percentage or ms ?
   > 
   > https://www.postgresql.org/docs/current/auto-explain.html
   > |log_analyze
   > ...
   > |When this parameter is on, per-plan-node timing occurs for all statements executed, whether or not they run long enough to actually get logged. This can have an extremely negative impact on performance. Turning off auto_explain.log_timing ameliorates the performance cost, at the price of obtaining less information.
   > 
   > |auto_explain.log_timing (boolean)
   > |auto_explain.log_timing controls whether per-node timing information is printed when an execution plan is logged; it's equivalent to the TIMING option of EXPLAIN. The overhead of repeatedly reading the system clock can slow down queries significantly on some systems, so it may be useful to set this parameter to off when only actual row counts, and not exact times, are needed. This parameter has no effect unless auto_explain.log_analyze is enabled. This parameter is on by default. Only superusers can change this setting.
   > 
   > I believe the cost actually varies significantly with the type of plan "node",
   > with "nested loops" incurring much higher overhead.
   > 
   > I think you could compare using explain(analyze) vs explain(analyze,timing
   > off).  While you're at it, compare without explain at all.
   > 
   > I suspect the overhead is inconsequential if you set log_timing=off and set
   > log_min_duration such that only the slowest queries are logged.
   > 
   > Then, you can manually run "explain (analyze,costs on)" on any problematic
   > queries to avoid interfering with production clients.
   > 
   > Justin
   > 
   
   You should also consider auto_explain.sample_rate: 
   auto_explain.sample_rate causes auto_explain to only explain a fraction 
   of the statements in each session. The default is 1, meaning explain all 
   the queries. In case of nested statements, either all will be explained 
   or none. Only superusers can change this setting.
   
   This option is available since 9.6
   
   Regards
   
   
   

4. Re: impact of auto explain on overall performance

   Jeff Janes <jeff.janes@gmail.com> — 2019-03-14T17:50:07Z

   On Thu, Mar 14, 2019 at 3:29 AM Stephan Schmidt <schmidt@dltmail.de> wrote:
   
   > Hello,
   >
   >
   >
   > i’m currently working on a high Performance Database and want to make sure
   > that whenever there are slow queries during regular operations i’ve got all
   > Information about the query in my logs. So auto_explain come to mind, but
   > the documentation explicitly states that it Comes at a cost. My Question
   > is, how big is the latency added by auto_explain in percentage or ms ?
   >
   
   You will have to measure it yourself and see.  It depends on your hardware,
   OS, and OS version, and PostgreSQL version.  And the nature of your
   queries.  If you have auto_explain.log_timing=on, then I find that large
   sorts are the worst impacted.  So if you have a lot of those, you should be
   careful.
   
   On older kernels, I would run with auto_explain.log_timing=off.  On newer
   kernels where you can read the clock from user-space, I run with
   auto_explain.log_timing=on.  I find the slowdown noticeable with careful
   investigation (around 3%, last time I carefully investigated it), but
   usually well worth paying to have actual data to work with when I find slow
   queries in the log.  I made a special role with auto_explain disabled for
   use with a few reporting queries with large sorts, both to circumvent the
   overhead and to avoid spamming the log with slow queries I already know
   about.
   
   Cheers,
   
   Jeff
   
   >
   

5. Re: impact of auto explain on overall performance

   Jeremy Schneider <schneider@ardentperf.com> — 2019-03-14T19:58:18Z

   On 3/14/19 00:29, Stephan Schmidt wrote:
   > i’m currently working on a high Performance Database and want to make
   > sure that whenever there are slow queries during regular operations i’ve
   > got all Information about the query in my logs. So auto_explain come to
   > mind, but the documentation explicitly states that it Comes at a cost.
   > My Question is, how big is the latency added by auto_explain in
   > percentage or ms ?
   
   One thought - what if the problem query is a 4ms query that just went to
   6ms but it's executed millions of times per second? That would create a
   150% increase to the load on the system.
   
   The approach I've had the most success with is to combine active session
   sampling (from pg_stat_activity) with pg_stat_statements (ideally with
   periodic snapshots) to identify problematic SQL statements, then use
   explain analyze after you've identified them.
   
   There are a handful of extensions on the internet that can do active
   session sampling for you, and I've seen a few scripts that can be put
   into a scheduler to capture snapshots of stats tables.
   
   Maybe something to consider in addition to the auto_explain stuff.
   
   -Jeremy
   
   -- 
   http://about.me/jeremy_schneider