Re: impact of auto explain on overall performance

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