Postgres-XC 1.1 Documentation
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# 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 all sessions by including `auto_explain` in 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.

> Note: The following description applies only to Postgres-XC

To log plans on Datanodes, you must preload this module in each Datanode. This module will log local plans of each node. For example, Coordinator log will include the plan for Coordinator only. Corresponding plan in Datanodes will be found in each Datanode's log.

## 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.

`auto_explain.log_verbose` (`boolean`)

  `auto_explain.log_verbose` causes `EXPLAIN VERBOSE` 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.

`auto_explain.log_buffers` (`boolean`)

  `auto_explain.log_buffers` causes `EXPLAIN (ANALYZE, BUFFERS)` 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. This parameter has no effect unless `auto_explain.log_analyze` parameter is set.

`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.

These parameters must be set in `postgresql.conf`. Typical usage might be:

    # postgresql.conf
    shared_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=# 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 `<itagaki.takahiro@oss.ntt.co.jp>`

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