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
