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.
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.
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'
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
Takahiro Itagaki <[email protected]>