F.3. auto_explain

The auto_explain module provides a means for logging execution plans of slow statements automatically, without having to run EXPLAIN by hand. This is especially helpful for tracking down un-optimized queries in large applications.

The module provides no SQL-accessible functions. To use it, simply load it into the server. You can load it into an individual session:

LOAD 'auto_explain';

(You must be superuser to do that.) More typical usage is to preload it into some or all sessions by including auto_explain in session_preload_libraries or shared_preload_libraries in postgresql.conf. Then you can track unexpectedly slow queries no matter when they happen. Of course there is a price in overhead for that.

F.3.1. Configuration Parameters

There are several configuration parameters that control the behavior of auto_explain. Note that the default behavior is to do nothing, so you must set at least auto_explain.log_min_duration if you want any results.

auto_explain.log_min_duration (integer)

auto_explain.log_min_duration is the minimum statement execution time, in milliseconds, that will cause the statement's plan to be logged. Setting this to zero logs all plans. Minus-one (the default) disables logging of plans. For example, if you set it to 250ms then all statements that run 250ms or longer will be logged. Only superusers can change this setting.

auto_explain.log_analyze (boolean)

auto_explain.log_analyze causes EXPLAIN ANALYZE output, rather than just EXPLAIN output, to be printed when an execution plan is logged. This parameter is off by default. Only superusers can change this setting.

Note: 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_buffers (boolean)

auto_explain.log_buffers controls whether buffer usage statistics are printed when an execution plan is logged; it's equivalent to the BUFFERS option of EXPLAIN. This parameter has no effect unless auto_explain.log_analyze is enabled. This parameter is off by default. Only superusers can change this setting.

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.

auto_explain.log_triggers (boolean)

auto_explain.log_triggers causes trigger execution statistics to be included when an execution plan is logged. This parameter has no effect unless auto_explain.log_analyze is enabled. This parameter is off by default. Only superusers can change this setting.

auto_explain.log_verbose (boolean)

auto_explain.log_verbose controls whether verbose details are printed when an execution plan is logged; it's equivalent to the VERBOSE option of EXPLAIN. This parameter is off by default. Only superusers can change this setting.

auto_explain.log_format (enum)

auto_explain.log_format selects the EXPLAIN output format to be used. The allowed values are text, xml, json, and yaml. The default is text. Only superusers can change this setting.

auto_explain.log_nested_statements (boolean)

auto_explain.log_nested_statements causes nested statements (statements executed inside a function) to be considered for logging. When it is off, only top-level query plans are logged. This parameter is off by default. Only superusers can change this setting.

auto_explain.sample_rate (real)

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.

In ordinary usage, these parameters are set in postgresql.conf, although superusers can alter them on-the-fly within their own sessions. Typical usage might be:

# postgresql.conf
session_preload_libraries = 'auto_explain'

auto_explain.log_min_duration = '3s'

F.3.2. Example

postgres=# LOAD 'auto_explain';
postgres=# SET auto_explain.log_min_duration = 0;
postgres=# SET auto_explain.log_analyze = true;
postgres=# SELECT count(*)
           FROM pg_class, pg_index
           WHERE oid = indrelid AND indisunique;

This might produce log output such as:

LOG:  duration: 3.651 ms  plan:
  Query Text: SELECT count(*)
              FROM pg_class, pg_index
              WHERE oid = indrelid AND indisunique;
  Aggregate  (cost=16.79..16.80 rows=1 width=0) (actual time=3.626..3.627 rows=1 loops=1)
    ->  Hash Join  (cost=4.17..16.55 rows=92 width=0) (actual time=3.349..3.594 rows=92 loops=1)
          Hash Cond: (pg_class.oid = pg_index.indrelid)
          ->  Seq Scan on pg_class  (cost=0.00..9.55 rows=255 width=4) (actual time=0.016..0.140 rows=255 loops=1)
          ->  Hash  (cost=3.02..3.02 rows=92 width=4) (actual time=3.238..3.238 rows=92 loops=1)
                Buckets: 1024  Batches: 1  Memory Usage: 4kB
                ->  Seq Scan on pg_index  (cost=0.00..3.02 rows=92 width=4) (actual time=0.008..3.187 rows=92 loops=1)
                      Filter: indisunique

F.3.3. Author

Takahiro Itagaki

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