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pg_find_plans: Find plans in pg_stat_plans based on arbitrary criteria.

Version: 1.0

Author: Peter Geoghegan

Introduction

pg_find_plans is an experimental submodule of pg_stat_plans. pg_stat_plans is a framework for understanding plan execution costs, that by design exposes some relatively low-level, extensible functionality, to be used as clients see fit - plan costs are simply tracked in a view that the module makes available. Query texts stored by the module can be explained on an ad-hoc basis (the module manages some corner-cases with that), and machine-readable explain texts are made available via a simple function-call API. For the most part, if users want to ask complicated questions about plans, they have to figure out their own way of doing so. Some users might be inclined to write queries like this, for example:

mydb=# select * from pg_stat_plans where query ilike '%select%mytable%';

This sort of approach is error-prone, rather limited in the types of questions that can be asked, and generally unpolished. The pg_find_plans submodule exists to provide users of pg_stat_plans with a higher-level, broadly useful set of functionality for storing machine-readable explain plans, and subsequently querying the plans in an arbitrary manner with confidence, to learn more about plan execution costs.

pg_find_plans is written in PL/Python and PL/PgSQL. It is intended to provide users with a better way to ask questions like "what are the execution costs of all plans tracked since last statistics reset that involve a sequential scan against mytable, and have more than 2 joins?". That might be written as:

mydb=# select
  join_count(json_plan),
  p.*
from
  pg_stat_plans p
  join
  stored_plans sp on (p.planid=sp.planid and p.userid=sp.userid and p.dbid=sp.dbid)
where
  from_our_database
and
  join_count(json_plan) > 2
and
  contains_node(json_plan, 'Seq Scan', 'mytable');
order by
  1 desc nulls last;

Users should have a high degree of confidence that their queries on plan's structure are free of detectable errors, and pg_find_plans ensures this by carefully sanitizing user input. For example, if the node of interest was specified as 'seq scan' above, the query would raise an error - to do any less might result in a false sense of security about the actual costs of plans that sequentially scan the table mytable, since the implementation might then naively ignore sequential scan nodes, as a case-sensitive comparison is used internally. In general, making the interface hard to use incorrectly is even more important than making it easy to use correctly.

Strictly speaking, pg_find_plans is nothing more than a simple set of functions for storing JSON explain texts of plans that appear as pg_stat_plans entries into a dedicated table, and subsequently parsing those plans to answer interesting questions using SQL. However, pg_find_plans is a module that is likely to make pg_stat_plans much more useful than it might otherwise be. pg_find_plans is by no means feature complete or especially polished. The author's ambitions for the tool are described under "limitations" below.

Sample queries

Sample queries are available from the file samples.sql, which is distributed with pg_find_plans in a subdirectory of pg_stat_plans. These give usage examples that are likely to answer questions interesting to DBAs running production PostgreSQL systems, including showing costs for plans involving particular tables or indexes, or showing plan costs organized by the number of joins involved in a query.

Installation

pg_find_plans has some dependencies above and beyond those of pg_stat_plans. This is one reason why it is not required to use pg_find_plans with pg_stat_plans. pg_find_plans requires:

  • pg_stat_plans + PostgreSQL. pg_find_plans will work with any version of Postgres that is supported by pg_stat_plans.
  • PL/PgSQL. Available by default on all Postgres versions that pg_stat_plans supports, so arguably its inclusion here is redundant.
  • PL/Python, based on Python version 2.6+. Since the Python standard library JSON decoder was introduced in that version, it is unlikely that pg_find_plans will ever support an earlier version of Python. pg_find_plans should work with any version of Python 2 after 2.6, but the Python 3 language variant is not currently supported.

Although the module is entirely written in SQL, PL/PgSQL and PL/Python, there is a PGXS Makefile supplied, which manages installation. If you are using a packaged PostgreSQL build and have pg_config available (and in your OS user's $PATH), the procedure is as follows:

cd pg_find_plans
make
make install

You may need to refer to platform-specific notes on installation issues that appear in the main pg_stat_plans README.

Setting up PostgreSQL

pg_find_plans objects must be installed in every database that they are required. pg_find_plans uses the PostgreSQL extension mechanism where available. To install on PostgreSQL versions 9.1+, execute the following SQL command:

mydb=# CREATE EXTENSION pg_find_plans;

Earlier releases (that lack the extension mechanism - in practice, this is limited to version 9.0) must install the module by executing the SQL script directly:

psql mydb -f pg_find_plans.sql

Making sure that new plans are available to search through

Finally, you'll need to figure out some way of maintaining the materialized plans table, stored_plans, since Postgres has no built-in way of scheduling arbitrary maintenance tasks. See "Suggested setup" below.

Objects made available by pg_stat_plans

The pg_find_plans module provides numerous database objects. Example usage for most of these objects is shown within samples.sql.

node_type enum

This enum is comprised of constants that exactly match node types within a non-text format Postgres EXPLAIN (as of Postgres 9.2 - versions 9.0 and 9.1 of Postgres have constants that are a strict subset of the 9.2 values). The following psql command will show all possible values:

mydb=# \dT+ node_type

command_type enum

See notes on contains_modifytable function below.

agg_strategy enum

See notes on contains_aggregate function below.

setop_strategy enum

See notes on contains_setop function below.

join_type enum

See notes on join_count function below.

stored_plans table

The stored_plans table is where plans for pg_stat_plans entries may be materialized to. Its definition is:

      Column       |       Type       | Modifiers
-------------------+------------------+-----------
 planid            | oid              | not null
 userid            | oid              | not null
 dbid              | oid              | not null
 json_plan         | text             |
 last_startup_cost | double precision | not null
 last_total_cost   | double precision | not null
 error_seen        | text             |
Indexes:
    "stored_plans_pkey" PRIMARY KEY, btree (planid, userid, dbid)

contains_node function

This function is used to find plan nodes from a Postgres JSON-format explain text. Its signature is:

contains_node(json_plan text, node node_type default null,
                      relation regclass default null)
  returns boolean

The function takes a single plan, specified by json_plan, and a node type, specified by node (which is an enum type with constants that correspond to the names of nodes as they appear within non-text format explain plans). Optionally, the user may specify a relation, which must refer to a relation (index or table) within the current database. Note that it is possible to query for 'Index Scan' and even 'Index Only Scan' against a table, and have a query show results for any of that table's indexes. Note that node may be given as null, indicating that all relation-scan nodes (plan leaf nodes) are of interest. This can be used to return costs for plans that involve a given relation in any way. For example:

select *
...
where contains_node(json_plan, relation := 'orders')

The boolean value returned by the function indicates if the JSON plan contains node(s) that fit. If a relation was specified, the plan must contain the relation in respect of the node in order to match. For example, if node was a 'Seq Scan', relation name might optionally be specified as 'mytable'. In that case, the function would only return true if the plan contained a sequential scan on 'mytable'. It would not be sufficient for the plan to contain just a sequential scan and some other reference (such as a 'Bitmap Heap Scan') to the relation 'mytable'.

contains_nodes function

This function is a convenience variant of contains_node, intended to be used with multiple relations rather than just a single one. Its signature is:

contains_nodes(json_plan text,
                      node node_type default null,
                      relations regclass[] default null)
  returns boolean

Again, if node is not specified, this indicates that all relation-scan nodes (plan leaf nodes) are of interest. Relations may also be null, indicating that all relations are of interest.

As with contains_node, the boolean value returned by contains_nodes indicates if a match occurred.

contains_modifytable function

This function is used to search for ModifyTable nodes based on Operation, and potentially, affected relation. The functions signature is:

contains_modifytable(json_plan text,
                        command command_type default null,
                        relation regclass default null)
  returns boolean

The boolean value returned by contains_modifytable indicates if a match occurred.

"command" can be any one of 'Insert', 'Update' or 'Delete'. Alternatively, if "command" is null, all ModifyTable nodes that operate on "relation" can be returned. If "relation" is null, all ModifyTable nodes with commands/operations that match "command" will cause the function to return true, regardless of what relation they apply to.

contains_aggregate function

This function is used to search for aggregate nodes based on "Strategy". The functions signature is:

contains_aggregate(json_plan text, strategy agg_strategy)
  returns boolean

The strategy specified by "strategy" can be any one of 'Aggregate', 'GroupAggregate' and 'HashAggregate'. Those who are used to the text explain format may be puzzled by this, regarding these various types of aggregates as distinct node types, rather than strategies of a single node. In fact, this scheme better reflects the actual structure of the code within the executor.

The boolean value returned by contains_aggregate indicates if a match occurred.

contains_setop function

This function is used to search for set operation nodes based on "Strategy". The functions signature is:

contains_setop(json_plan text, strategy setop_strategy)
  returns boolean

The strategy specified by "strategy" can be either 'Sorted' (typically seen when the set operation must eliminate duplicates) or 'Hashed' (typically seen for union all set operations).

Note that set operations are not guaranteed to produce a plan with a SetOp node. For example, it is possible for the planner to produce an Append node.

The boolean value returned by contains_setop indicates if a match occurred.

join_count function

join_count returns the number of joins within a JSON-format explain text. Its signature is:

join_count(json_plan text, count join_type default null)
  returns integer

The count_type argument limits joins counted to one of several types of join. These are: 'Inner', 'Left', 'Full', 'Right', 'Semi', 'Anti' and 'Outer' (i.e. any one of 'Left', 'Full', 'Right'). If count_type is null, all join types are considered of interest. Note that if a particular join strategy type is of interest (such as a 'Nested Loop' join), nodes of that type can be found with the contains_node function. Naturally, all JSON plan parsing pg_find_plans functions can be usefully combined within a query predicate.

materialize_plans function

This function aggregates JSON explain texts for each entry within pg_stat_plans:

materialize_plans(ignore_costs boolean default true)
  returns void

It lazily explains only those entries of pg_stat_plans that don't already have an entry. It is the intended infrastructure through which DBAs can materialize plans tracked by pg_stat_plans asynchronously. See notes in "Suggested setup" below, including details of the "ignore_costs" argument. "ignore_costs" has the function not consider differing costs (between materialized JSON plans and the pg_stat_plans view) as a reason for re-EXPLAINing.

trim_stored_plans function

This function "trims" (i.e. deletes) entries within stored_plans that are no longer present within pg_stat_plans, typically due to application churn and pressure on pg_stat_plans' fixed-sized cache removing marginal entries/plans. Its signature is:

trim_stored_plans()
  returns void

Typically usage is described below, under "Suggested setup".

Suggested setup

The need to maintain materialized plans within a dedicated table naturally implies some overhead. However, since in practice the actual entries within pg_stat_plans can be expected to be reasonably static, pg_find_plans' ability to lazily explain only new entries that lack an explain text makes that overhead likely to be quite modest in practice.

It is assumed that it is not absolutely critical that the latest plan texts are always available. samples.sql contains a query that will show entries as yet unprocessed by pg_find_plans. In order to maintain materialized JSON plan texts, the following SQL should be executed at regular intervals, such as every 15 minutes:

select materialize_plans();

This will only explain and store new plans that have not yet been processed. It may also be useful to call the function in this manner less frequently, perhaps once a day:

select materialize_plans(ignore_costs:=false);

This will cause materialize_plans() to lazily skip existing entries if and only if both the startup cost and total cost of each query, as stored within the stored_plans materialization table, and the last startup and total costs pg_stat_plans saw for a given query match exactly. In other words, this should be done out of a concern for keeping the plan costs for each plan, as represented in the stored JSON plan texts, consistent with their actual current values as measured by the planner. It is quite possible for a plan's costs to change, without the planner producing a substantively different plan, due to alterations in planner cost constants (i.e. server settings), new statistics becoming available after ANALYZE is run, and so on.

N.B.: This SQL needs to be called from each and every database of your PostgreSQL installation that is of interest. pg_find_plans assumes that only the current database is of interest, despite the fact that pg_stat_plans aggregates statistics for the entire installation.

You may also wish to delete unused plans within the stored_plans table, by calling trim_stored_plans() at regular intervals. It may make sense to do so less often, perhaps at the same time as performing a materialize_plans() that does not ignore startup_costs/total_cost as a reason to refresh plan texts.

Example crontab

To follow the advice above, the following crontab can be used:

# Every 15 minutes do small job (though not between 4 and 5)
*/15 0-3,5-23 * * * psql -d mydb -f /path/to/materialize_ignore.sql
# 4:15, 4:30, 4:45 - still not worried about costs + trimming
15-45/15 4 * * * psql -d mydb -f /path/to/materialize_ignore.sql
# At 4:00, perform extra work. These operations will block each other, so
# we've avoiding doing an "ignore" and "no ignore + trim" materialize (or,
# indeed, any type of materialize) at the same time. Note that
# materialize_plans() acquires a NOWAIT table-level lock, and so
# materialization requests cannot pile-up due to unusually slow planning.
0 4 * * * psql -d mydb -f /path/to/materialize_no_ignore_trim.sql

The ability to manipulate the scheduler from an SQL interface may be desired. The PgAgent utility offers this capability, providing scheduler functionality that can be manipulated through Postgres (i.e. schedules are stored in regular tables) on all supported platforms. PgAgent is available from:

http://www.pgadmin.org/download/pgagent.php

Limitations

Unlike pg_stat_plans, pg_find_plans is an experimental tool, and expedient hack; it relies on the stored query text within pg_stat_plans to reproduce the same plan originally observed asynchronously, often some number of minutes later. In general, the query text cannot reproduce the EXPLAIN with a high degree of reliability, since the planner's preferences may have changed for various reasons noted separately in the main pg_stat_plans README. We may easily encounter a scenario in which we have no reasonable expectation of producing an EXPLAIN ever working for some number of entries. While pg_stat_plan's fingerprinting mechanism ensures that the JSON explain text is consistent with the plan originally seen for a pg_stat_plans entry, there may be scenarios in which an unreasonably high number of entries can never be explained. Most obviously, pg_find_plans will never work with prepared queries, and all PLs use prepared queries internally. For that reason, you may wish to only run pg_find_plans when pg_stat_plans.track is set to 'top'.

Proof-of-concept

There is a fairly obvious basic, alternative approach that could be taken to implement pg_find_plans' functionality in a much better-principled way: an explain JSON text could be produced synchronously, within an ExecutorEnd() hook, in the style of contrib/auto_explain. Since that process can be performed directly on a queryDesc (the data structure that encapsulates everything that the executor needs to execute a query, in particular, its plan), the fragility of taking a query text as a proxy for a plan - the basic problem with pg_find_plans - is entirely eliminated.

This is a non-trivial undertaking, though. contrib/auto_explain simply logs slow-running plans. pg_stat_plans would have to store plans in way that readily facilitated querying the plans, while imposing minimal overhead on statement execution. For example, it would perhaps be necessary to allocate some fixed amount of additional shared memory to store JSON explain texts, while avoiding truncating these texts whenever possible, since truncation makes the texts completely useless. Developing a mechanism for lazily and efficiently storing those JSON texts in an arbitrarily sized area is surprisingly involved.

Future direction

In the future, this approach may be taken, and pg_find_plans may become largely obsolete, at least as a mechanism for materializing JSON plans. It is far from obvious how much demand there is for pg_find_plans type functionality at present, and due to constraints on the author's time, these ideas are only ever likely to be pursued due to strong demand from 2ndQuadrant support customers, or the PostgreSQL community at large. pg_find_plans can be thought of as a proof-of-concept for features that pg_stat_plans could directly offer polished, mature versions of, given sufficient attention. It can also be thought of as a springboard for ideas about how we can query the structure of the entirety of your database's plans. If it is possible to ask arbitrary questions about the structure of plans used in production, what questions are actually interesting? How can we present that information in a way that is actually actionable or immediately useful?

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