The pg_pathman module provides optimized partitioning mechanism and functions to manage partitions.
The extension is compatible with PostgreSQL 9.5+.
Partitioning means splitting one large table into smaller pieces. Each row in such table is moved to a single partition according to the partitioning key. PostgreSQL supports partitioning via table inheritance: each partition must be created as a child table with CHECK CONSTRAINT. For example:
CREATE TABLE test (id SERIAL PRIMARY KEY, title TEXT);
CREATE TABLE test_1 (CHECK ( id >= 100 AND id < 200 )) INHERITS (test);
CREATE TABLE test_2 (CHECK ( id >= 200 AND id < 300 )) INHERITS (test);Despite the flexibility, this approach forces the planner to perform an exhaustive search and to check constraints on each partition to determine whether it should be present in the plan or not. Large amount of partitions may result in significant planning overhead.
The pg_pathman module features partition managing functions and optimized planning mechanism which utilizes knowledge of the partitions' structure. It stores partitioning configuration in the pathman_config table; each row contains a single entry for a partitioned table (relation name, partitioning column and its type). During the initialization stage the pg_pathman module caches some information about child partitions in the shared memory, which is used later for plan construction. Before a SELECT query is executed, pg_pathman traverses the condition tree in search of expressions like:
VARIABLE OP CONST
where VARIABLE is a partitioning key, OP is a comparison operator (supported operators are =, <, <=, >, >=), CONST is a scalar value. For example:
WHERE id = 150Based on the partitioning type and condition's operator, pg_pathman searches for the corresponding partitions and builds the plan. Currently pg_pathman supports two partitioning schemes:
- RANGE - maps rows to partitions using partitioning key ranges assigned to each partition. Optimization is achieved by using the binary search algorithm;
- HASH - maps rows to partitions using a generic hash function.
More interesting features are yet to come. Stay tuned!
- HASH and RANGE partitioning schemes;
- Both automatic and manual partition management;
- Support for integer, floating point, date and other types, including domains;
- Effective query planning for partitioned tables (JOINs, subselects etc);
RuntimeAppend&RuntimeMergeAppendcustom plan nodes to pick partitions at runtime;PartitionFilter: an efficient drop-in replacement for INSERT triggers;- Automatic partition creation for new INSERTed data (only for RANGE partitioning);
- Improved
COPY FROM\TOstatement that is able to insert rows directly into partitions; - UPDATE triggers generation out of the box (will be replaced with custom nodes too);
- User-defined callbacks for partition creation event handling;
- Non-blocking concurrent table partitioning;
- FDW support (foreign partitions);
- Various GUC toggles and configurable settings.
- Implement LIST partitioning scheme;
- Optimize hash join (both tables are partitioned by join key).
To install pg_pathman, execute this in the module's directory:
make install USE_PGXS=1Modify the shared_preload_libraries parameter in postgresql.conf as following:
shared_preload_libraries = 'pg_pathman'
Important:
pg_pathmanmay have conflicts with some other extensions which uses the same hook functions. For example,pg_pathmanusesProcessUtility_hookhook to handle COPY queries for partitioned tables. And it could sometimes interfere withpg_stat_statementsextension which uses the same hook. In this case try to list libraries in certain order:shared_preload_libraries = 'pg_pathman, pg_stat_statements'
It is essential to restart the PostgreSQL instance. After that, execute the following query in psql:
CREATE EXTENSION pg_pathman;Done! Now it's time to setup your partitioning schemes.
Important: Don't forget to set the
PG_CONFIGvariable in case you want to testpg_pathmanon a custom build of PostgreSQL. Read more here.
create_hash_partitions(relation REGCLASS,
attribute TEXT,
partitions_count INTEGER,
partition_name TEXT DEFAULT NULL,
partition_data BOOLEAN DEFAULT TRUE)Performs HASH partitioning for relation by integer key attribute. The partitions_count parameter specifies the number of partitions to create; it cannot be changed afterwards. If partition_data is true then all the data will be automatically copied from the parent table to partitions. Note that data migration may took a while to finish and the table will be locked until transaction commits. See partition_table_concurrently() for a lock-free way to migrate data. Partition creation callback is invoked for each partition if set beforehand (see set_part_init_callback()).
create_range_partitions(relation REGCLASS,
attribute TEXT,
start_value ANYELEMENT,
p_interval ANYELEMENT,
p_count INTEGER DEFAULT NULL
partition_data BOOLEAN DEFAULT TRUE)
create_range_partitions(relation REGCLASS,
attribute TEXT,
start_value ANYELEMENT,
p_interval INTERVAL,
p_count INTEGER DEFAULT NULL,
partition_data BOOLEAN DEFAULT TRUE)Performs RANGE partitioning for relation by partitioning key attribute. start_value argument specifies initial value, interval sets the range of values in a single partition, count is the number of premade partitions (if not set then pathman tries to determine it based on attribute values). Partition creation callback is invoked for each partition if set beforehand.
create_partitions_from_range(relation REGCLASS,
attribute TEXT,
start_value ANYELEMENT,
end_value ANYELEMENT,
p_interval ANYELEMENT,
partition_data BOOLEAN DEFAULT TRUE)
create_partitions_from_range(relation REGCLASS,
attribute TEXT,
start_value ANYELEMENT,
end_value ANYELEMENT,
p_interval INTERVAL,
partition_data BOOLEAN DEFAULT TRUE)Performs RANGE-partitioning from specified range for relation by partitioning key attribute. Partition creation callback is invoked for each partition if set beforehand.
partition_table_concurrently(relation REGCLASS,
batch_size INTEGER DEFAULT 1000,
sleep_time FLOAT8 DEFAULT 1.0)Starts a background worker to move data from parent table to partitions. The worker utilizes short transactions to copy small batches of data (up to 10K rows per transaction) and thus doesn't significantly interfere with user's activity. If the worker is unable to lock rows of a batch, it sleeps for sleep_time seconds before the next attempt and tries again up to 60 times, and quits if it's still unable to lock the batch.
stop_concurrent_part_task(relation REGCLASS)Stops a background worker performing a concurrent partitioning task. Note: worker will exit after it finishes relocating a current batch.
create_hash_update_trigger(parent REGCLASS)Creates the trigger on UPDATE for HASH partitions. The UPDATE trigger isn't created by default because of the overhead. It's useful in cases when the key attribute might change.
create_range_update_trigger(parent REGCLASS)Same as above, but for a RANGE-partitioned table.
split_range_partition(partition REGCLASS,
split_value ANYELEMENT,
partition_name TEXT DEFAULT NULL)Split RANGE partition in two by split_value. Partition creation callback is invoked for a new partition if available.
merge_range_partitions(partition1 REGCLASS, partition2 REGCLASS)Merge two adjacent RANGE partitions. First, data from partition2 is copied to partition1, then partition2 is removed.
append_range_partition(parent REGCLASS,
partition_name TEXT DEFAULT NULL,
tablespace TEXT DEFAULT NULL)Append new RANGE partition with pathman_config.range_interval as interval.
prepend_range_partition(parent REGCLASS,
partition_name TEXT DEFAULT NULL,
tablespace TEXT DEFAULT NULL)Prepend new RANGE partition with pathman_config.range_interval as interval.
add_range_partition(relation REGCLASS,
start_value ANYELEMENT,
end_value ANYELEMENT,
partition_name TEXT DEFAULT NULL,
tablespace TEXT DEFAULT NULL)Create new RANGE partition for relation with specified range bounds.
drop_range_partition(partition TEXT, delete_data BOOLEAN DEFAULT TRUE)Drop RANGE partition and all of its data if delete_data is true.
attach_range_partition(relation REGCLASS,
partition REGCLASS,
start_value ANYELEMENT,
end_value ANYELEMENT)Attach partition to the existing RANGE-partitioned relation. The attached table must have exactly the same structure as the parent table, including the dropped columns. Partition creation callback is invoked if set (see pathman_config_params).
detach_range_partition(partition REGCLASS)Detach partition from the existing RANGE-partitioned relation.
disable_pathman_for(relation TEXT)Permanently disable pg_pathman partitioning mechanism for the specified parent table and remove the insert trigger if it exists. All partitions and data remain unchanged.
drop_partitions(parent REGCLASS,
delete_data BOOLEAN DEFAULT FALSE)Drop partitions of the parent table (both foreign and local relations). If delete_data is false, the data is copied to the parent table first. Default is false.
set_enable_parent(relation REGCLASS, value BOOLEAN)Include/exclude parent table into/from query plan. In original PostgreSQL planner parent table is always included into query plan even if it's empty which can lead to additional overhead. You can use disable_parent() if you are never going to use parent table as a storage. Default value depends on the partition_data parameter that was specified during initial partitioning in create_range_partitions() or create_partitions_from_range() functions. If the partition_data parameter was true then all data have already been migrated to partitions and parent table disabled. Otherwise it is enabled.
set_auto(relation REGCLASS, value BOOLEAN)Enable/disable auto partition propagation (only for RANGE partitioning). It is enabled by default.
set_init_callback(relation REGCLASS, callback REGPROC DEFAULT 0)Set partition creation callback to be invoked for each attached or created partition (both HASH and RANGE). The callback must have the following signature: part_init_callback(args JSONB) RETURNS VOID. Parameter arg consists of several fields whose presence depends on partitioning type:
/* RANGE-partitioned table abc (child abc_4) */
{
"parent": "abc",
"parttype": "2",
"partition": "abc_4",
"range_max": "401",
"range_min": "301"
}
/* HASH-partitioned table abc (child abc_0) */
{
"parent": "abc",
"parttype": "1",
"partition": "abc_0"
}CREATE TABLE IF NOT EXISTS pathman_config (
partrel REGCLASS NOT NULL PRIMARY KEY,
attname TEXT NOT NULL,
parttype INTEGER NOT NULL,
range_interval TEXT,
CHECK (parttype IN (1, 2)) /* check for allowed part types */ );This table stores a list of partitioned tables.
CREATE TABLE IF NOT EXISTS pathman_config_params (
partrel REGCLASS NOT NULL PRIMARY KEY,
enable_parent BOOLEAN NOT NULL DEFAULT TRUE,
auto BOOLEAN NOT NULL DEFAULT TRUE,
init_callback REGPROCEDURE NOT NULL DEFAULT 0);This table stores optional parameters which override standard behavior.
-- helper SRF function
CREATE OR REPLACE FUNCTION show_concurrent_part_tasks()
RETURNS TABLE (
userid REGROLE,
pid INT,
dbid OID,
relid REGCLASS,
processed INT,
status TEXT)
AS 'pg_pathman', 'show_concurrent_part_tasks_internal'
LANGUAGE C STRICT;
CREATE OR REPLACE VIEW pathman_concurrent_part_tasks
AS SELECT * FROM show_concurrent_part_tasks();This view lists all currently running concurrent partitioning tasks.
-- helper SRF function
CREATE OR REPLACE FUNCTION show_partition_list()
RETURNS TABLE (
parent REGCLASS,
partition REGCLASS,
parttype INT4,
partattr TEXT,
range_min TEXT,
range_max TEXT)
AS 'pg_pathman', 'show_partition_list_internal'
LANGUAGE C STRICT;
CREATE OR REPLACE VIEW pathman_partition_list
AS SELECT * FROM show_partition_list();This view lists all existing partitions, as well as their parents and range boundaries (NULL for HASH partitions).
pg_pathman provides a couple of custom plan nodes which aim to reduce execution time, namely:
RuntimeAppend(overridesAppendplan node)RuntimeMergeAppend(overridesMergeAppendplan node)PartitionFilter(drop-in replacement for INSERT triggers)
PartitionFilter acts as a proxy node for INSERT's child scan, which means it can redirect output tuples to the corresponding partition:
EXPLAIN (COSTS OFF)
INSERT INTO partitioned_table
SELECT generate_series(1, 10), random();
QUERY PLAN
-----------------------------------------
Insert on partitioned_table
-> Custom Scan (PartitionFilter)
-> Subquery Scan on "*SELECT*"
-> Result
(4 rows)RuntimeAppend and RuntimeMergeAppend have much in common: they come in handy in a case when WHERE condition takes form of:
VARIABLE OP PARAM
This kind of expressions can no longer be optimized at planning time since the parameter's value is not known until the execution stage takes place. The problem can be solved by embedding the WHERE condition analysis routine into the original Append's code, thus making it pick only required scans out of a whole bunch of planned partition scans. This effectively boils down to creation of a custom node capable of performing such a check.
There are at least several cases that demonstrate usefulness of these nodes:
/* create table we're going to partition */
CREATE TABLE partitioned_table(id INT NOT NULL, payload REAL);
/* insert some data */
INSERT INTO partitioned_table
SELECT generate_series(1, 1000), random();
/* perform partitioning */
SELECT create_hash_partitions('partitioned_table', 'id', 100);
/* create ordinary table */
CREATE TABLE some_table AS SELECT generate_series(1, 100) AS VAL;id = (select ... limit 1)
EXPLAIN (COSTS OFF, ANALYZE) SELECT * FROM partitioned_table
WHERE id = (SELECT * FROM some_table LIMIT 1);
QUERY PLAN
----------------------------------------------------------------------------------------------------
Custom Scan (RuntimeAppend) (actual time=0.030..0.033 rows=1 loops=1)
InitPlan 1 (returns $0)
-> Limit (actual time=0.011..0.011 rows=1 loops=1)
-> Seq Scan on some_table (actual time=0.010..0.010 rows=1 loops=1)
-> Seq Scan on partitioned_table_70 partitioned_table (actual time=0.004..0.006 rows=1 loops=1)
Filter: (id = $0)
Rows Removed by Filter: 9
Planning time: 1.131 ms
Execution time: 0.075 ms
(9 rows)
/* disable RuntimeAppend node */
SET pg_pathman.enable_runtimeappend = f;
EXPLAIN (COSTS OFF, ANALYZE) SELECT * FROM partitioned_table
WHERE id = (SELECT * FROM some_table LIMIT 1);
QUERY PLAN
----------------------------------------------------------------------------------
Append (actual time=0.196..0.274 rows=1 loops=1)
InitPlan 1 (returns $0)
-> Limit (actual time=0.005..0.005 rows=1 loops=1)
-> Seq Scan on some_table (actual time=0.003..0.003 rows=1 loops=1)
-> Seq Scan on partitioned_table_0 (actual time=0.014..0.014 rows=0 loops=1)
Filter: (id = $0)
Rows Removed by Filter: 6
-> Seq Scan on partitioned_table_1 (actual time=0.003..0.003 rows=0 loops=1)
Filter: (id = $0)
Rows Removed by Filter: 5
... /* more plans follow */
Planning time: 1.140 ms
Execution time: 0.855 ms
(306 rows)id = ANY (select ...)
EXPLAIN (COSTS OFF, ANALYZE) SELECT * FROM partitioned_table
WHERE id = any (SELECT * FROM some_table limit 4);
QUERY PLAN
-----------------------------------------------------------------------------------------------------------
Nested Loop (actual time=0.025..0.060 rows=4 loops=1)
-> Limit (actual time=0.009..0.011 rows=4 loops=1)
-> Seq Scan on some_table (actual time=0.008..0.010 rows=4 loops=1)
-> Custom Scan (RuntimeAppend) (actual time=0.002..0.004 rows=1 loops=4)
-> Seq Scan on partitioned_table_70 partitioned_table (actual time=0.001..0.001 rows=10 loops=1)
-> Seq Scan on partitioned_table_26 partitioned_table (actual time=0.002..0.003 rows=9 loops=1)
-> Seq Scan on partitioned_table_27 partitioned_table (actual time=0.001..0.002 rows=20 loops=1)
-> Seq Scan on partitioned_table_63 partitioned_table (actual time=0.001..0.002 rows=9 loops=1)
Planning time: 0.771 ms
Execution time: 0.101 ms
(10 rows)
/* disable RuntimeAppend node */
SET pg_pathman.enable_runtimeappend = f;
EXPLAIN (COSTS OFF, ANALYZE) SELECT * FROM partitioned_table
WHERE id = any (SELECT * FROM some_table limit 4);
QUERY PLAN
-----------------------------------------------------------------------------------------
Nested Loop Semi Join (actual time=0.531..1.526 rows=4 loops=1)
Join Filter: (partitioned_table.id = some_table.val)
Rows Removed by Join Filter: 3990
-> Append (actual time=0.190..0.470 rows=1000 loops=1)
-> Seq Scan on partitioned_table (actual time=0.187..0.187 rows=0 loops=1)
-> Seq Scan on partitioned_table_0 (actual time=0.002..0.004 rows=6 loops=1)
-> Seq Scan on partitioned_table_1 (actual time=0.001..0.001 rows=5 loops=1)
-> Seq Scan on partitioned_table_2 (actual time=0.002..0.004 rows=14 loops=1)
... /* 96 scans follow */
-> Materialize (actual time=0.000..0.000 rows=4 loops=1000)
-> Limit (actual time=0.005..0.006 rows=4 loops=1)
-> Seq Scan on some_table (actual time=0.003..0.004 rows=4 loops=1)
Planning time: 2.169 ms
Execution time: 2.059 ms
(110 rows)NestLoopinvolving a partitioned table, which is omitted since it's occasionally shown above.
In case you're interested, you can read more about custom nodes at Alexander Korotkov's blog.
- You can easily add partition column containing the names of the underlying partitions using the system attribute called tableoid:
SELECT tableoid::regclass AS partition, * FROM partitioned_table;-
Though indices on a parent table aren't particularly useful (since it's supposed to be empty), they act as prototypes for indices on partitions. For each index on the parent table,
pg_pathmanwill create a similar index on every partition. -
All running concurrent partitioning tasks can be listed using the
pathman_concurrent_part_tasksview:
SELECT * FROM pathman_concurrent_part_tasks;
userid | pid | dbid | relid | processed | status
--------+------+-------+-------+-----------+---------
dmitry | 7367 | 16384 | test | 472000 | working
(1 row)pathman_partition_listin conjunction withdrop_range_partition()can be used to drop RANGE partitions in a more flexible way compared to good oldDROP TABLE:
SELECT drop_range_partition(partition, false) /* move data to parent */
FROM pathman_partition_list
WHERE parent = 'part_test'::regclass AND range_min::int < 500;
NOTICE: 1 rows copied from part_test_11
NOTICE: 100 rows copied from part_test_1
NOTICE: 100 rows copied from part_test_2
drop_range_partition
----------------------
dummy_test_11
dummy_test_1
dummy_test_2
(3 rows)- You can turn foreign tables into partitions using the
attach_range_partition()function. Rows that were meant to be inserted into parent will be redirected to foreign partitions (as usual, PartitionFilter will be involved), though by default it is prohibited to insert rows into partitions provided not bypostgres_fdw. Only superuser is allowed to setpg_pathman.insert_into_fdwGUC variable.
Consider an example of HASH partitioning. First create a table with some integer column:
CREATE TABLE items (
id SERIAL PRIMARY KEY,
name TEXT,
code BIGINT);
INSERT INTO items (id, name, code)
SELECT g, md5(g::text), random() * 100000
FROM generate_series(1, 100000) as g;Now run the create_hash_partitions() function with appropriate arguments:
SELECT create_hash_partitions('items', 'id', 100);This will create new partitions and move the data from parent to partitions.
Here's an example of the query performing filtering by partitioning key:
SELECT * FROM items WHERE id = 1234;
id | name | code
------+----------------------------------+------
1234 | 81dc9bdb52d04dc20036dbd8313ed055 | 1855
(1 row)
EXPLAIN SELECT * FROM items WHERE id = 1234;
QUERY PLAN
------------------------------------------------------------------------------------
Append (cost=0.28..8.29 rows=0 width=0)
-> Index Scan using items_34_pkey on items_34 (cost=0.28..8.29 rows=0 width=0)
Index Cond: (id = 1234)Notice that the Append node contains only one child scan which corresponds to the WHERE clause.
Important: pay attention to the fact that
pg_pathmanexcludes the parent table from the query plan.
To access parent table use ONLY modifier:
EXPLAIN SELECT * FROM ONLY items;
QUERY PLAN
------------------------------------------------------
Seq Scan on items (cost=0.00..0.00 rows=1 width=45)Consider an example of RANGE partitioning. Let's create a table containing some dummy logs:
CREATE TABLE journal (
id SERIAL,
dt TIMESTAMP NOT NULL,
level INTEGER,
msg TEXT);
-- similar index will also be created for each partition
CREATE INDEX ON journal(dt);
-- generate some data
INSERT INTO journal (dt, level, msg)
SELECT g, random() * 6, md5(g::text)
FROM generate_series('2015-01-01'::date, '2015-12-31'::date, '1 minute') as g;Run the create_range_partitions() function to create partitions so that each partition would contain the data for one day:
SELECT create_range_partitions('journal', 'dt', '2015-01-01'::date, '1 day'::interval);It will create 365 partitions and move the data from parent to partitions.
New partitions are appended automaticaly by insert trigger, but it can be done manually with the following functions:
-- add new partition with specified range
SELECT add_range_partition('journal', '2016-01-01'::date, '2016-01-07'::date);
-- append new partition with default range
SELECT append_range_partition('journal');The first one creates a partition with specified range. The second one creates a partition with default interval and appends it to the partition list. It is also possible to attach an existing table as partition. For example, we may want to attach an archive table (or even foreign table from another server) for some outdated data:
CREATE FOREIGN TABLE journal_archive (
id INTEGER NOT NULL,
dt TIMESTAMP NOT NULL,
level INTEGER,
msg TEXT)
SERVER archive_server;
SELECT attach_range_partition('journal', 'journal_archive', '2014-01-01'::date, '2015-01-01'::date);Important: the definition of the attached table must match the one of the existing partitioned table, including the dropped columns.
To merge to adjacent partitions, use the merge_range_partitions() function:
SELECT merge_range_partitions('journal_archive', 'journal_1');To split partition by value, use the split_range_partition() function:
SELECT split_range_partition('journal_366', '2016-01-03'::date);To detach partition, use the detach_range_partition() function:
SELECT detach_range_partition('journal_archive');Here's an example of the query performing filtering by partitioning key:
SELECT * FROM journal WHERE dt >= '2015-06-01' AND dt < '2015-06-03';
id | dt | level | msg
--------+---------------------+-------+----------------------------------
217441 | 2015-06-01 00:00:00 | 2 | 15053892d993ce19f580a128f87e3dbf
217442 | 2015-06-01 00:01:00 | 1 | 3a7c46f18a952d62ce5418ac2056010c
217443 | 2015-06-01 00:02:00 | 0 | 92c8de8f82faf0b139a3d99f2792311d
...
(2880 rows)
EXPLAIN SELECT * FROM journal WHERE dt >= '2015-06-01' AND dt < '2015-06-03';
QUERY PLAN
------------------------------------------------------------------
Append (cost=0.00..58.80 rows=0 width=0)
-> Seq Scan on journal_152 (cost=0.00..29.40 rows=0 width=0)
-> Seq Scan on journal_153 (cost=0.00..29.40 rows=0 width=0)
(3 rows)There are several user-accessible GUC variables designed to toggle the whole module or specific custom nodes on and off:
pg_pathman.enable--- disable (or enable)pg_pathmancompletelypg_pathman.enable_runtimeappend--- toggleRuntimeAppendcustom node on\offpg_pathman.enable_runtimemergeappend--- toggleRuntimeMergeAppendcustom node on\offpg_pathman.enable_partitionfilter--- togglePartitionFiltercustom node on\offpg_pathman.enable_auto_partition--- toggle automatic partition creation on\off (per session)pg_pathman.insert_into_fdw--- allow INSERTs into various FDWs(disabled | postgres | any_fdw)pg_pathman.override_copy--- toggle COPY statement hooking on\off
To permanently disable pg_pathman for some previously partitioned table, use the disable_pathman_for() function:
SELECT disable_pathman_for('range_rel');All sections and data will remain unchanged and will be handled by the standard PostgreSQL inheritance mechanism.
##Feedback Do not hesitate to post your issues, questions and new ideas at the issues page.
Ildar Musin i.musin@postgrespro.ru Postgres Professional Ltd., Russia Alexander Korotkov a.korotkov@postgrespro.ru Postgres Professional Ltd., Russia Dmitry Ivanov d.ivanov@postgrespro.ru Postgres Professional Ltd., Russia