What is User Defined Partitioning (UDP)?
All tables in Treasure Data are partitioned based on the time column. This is why queries that use TD_TIME_RANGE or similar predicates on the time column are efficient in Treasure Data. Presto can eliminate partitions that fall outside the specified time range without reading them.
User-defined partitioning (UDP) provides hash partitioning for a table on one or more columns in addition to the time column. A query that filters on the set of columns used as user-defined partitioning keys can be more efficient, because Presto can skip scanning partitions that have matching values on that set of columns.
Use Cases for UDP
Used properly, UDP can lead to more efficient lookup and aggregation queries, as well as join queries, where the lookup and aggregations are based on one or more specific columns.
UDP may add the most value when records are to be filtered or joined frequently on by non-time attributes such as:
- a customer's ID, first name+last name+birth date, gender, or other profile values or flags
- a product's SKU number, bar code, manufacturer, or other exact-match attributes
- an address's country code; city, state or province; or postal code
NOTE: Performance benefits become more significant on tables with >100M rows.
UDP can help with these Presto query types:
- "Needle-in-a-Haystack" lookup on the partition key
- Aggregations on the partition key
- Very large joins on partition keys used in tables on both sides of the join
Benefits may also be limited when used with more complex queries-- the query optimizer may not always apply UDP in cases where it can be beneficial.
Basic UDP Usage
CREATE TABLE Syntax for UDP
To create a UDP table, you must use CREATE TABLE with the attributes bucketed_on to identify the bucketing keys, and bucket_count for the number of buckets. The max_file_size and max_time_range values are optional.
CREATE TABLE table_name WITH
(bucketed_on = array['col1' [, 'col2']... ] , bucket_count = n
[, max_file_size = 'nnnMB'] [, max_time_range='nnd'])
[AS SELECT ... ];
For bucket_count the recommended value is 512.
create table customers_p with
(bucketed_on = array['col1' [, 'col2']... ] ,
If not specified, max_file_size will default to 256MB partitions and max_time_range to 1d or 24 hours for time partitioning.
Choosing Bucketing Columns for UDP
Supported TD data types for UDP partition keys include int, long and string.
Choose a set of one or more columns used widely to select data for analysis-- that is, one frequently used to look up results, drill down to details, or aggregate data. For example:
- Country + State/Province + City
- Customer first name + last name + date of birth
Choose a column or set of columns that have high cardinality (relative to the number of buckets), and which are frequently used with equality predicates. For example:
- Unique values, for example an email address or account number
- Non-unique but high-cardinality columns with relatively even distribution, for example date of birth
Checking for and addressing data skew
Performance will be inconsistent if the number of rows in each bucket are not roughly equal -- for example, if you partition on US zip/postal code, urban postal codes will have more customers than rural.
To help determine bucket count and partition size, run a SQL query that identifies distinct key column combinations and counts their occurrences. For example:
select (concat(key_column1 [, '|', key_column2])), count(*) from tbl ...
group by 1;
If the counts across different buckets are roughly comparable, your data is not skewed.
For consistent results, choose a combination of columns where the distribution is roughly equal.
Note: If you do decide to use partitioning keys that do not produce an even distribution, see "Tip: Improving Performance with Skewed Data."
INSERT/INSERT OVERWRITE into Partitioned Tables
INSERT and INSERT OVERWRITE with partitioned tables work the same as with other tables. You can create an empty UDP table and then insert data into it the usual way. The resulting data will be partitioned.
Partitioning an Existing Table
Tables must be configured for UDP when first created. For an existing table, you must create a copy of the table with UDP options configured and copy the rows over. To do this use a CTAS from the source table.
drop table if exists customer_p; create table customer_p with (bucketed_on = array['customer_id'],
bucket_count = 512) as select * from customer
When partitioning an existing table:
- Creating a partitioned version of a very large table is likely to take hours or days. Consult with TD support to make sure you can run this operation to completion.
- If the source table is continuing to receive updates, you will have to update it further with SQL like:
INSERT INTO customer_p SELECT * FROM customer WHERE...
Creating and Using UDP Tables: Examples
Create a partitioned copy of customer table named customer_p, to speed up lookups by customer_id;
drop table if exists customer_p; create table customer_p with (bucketed_on = array['customer_id'], bucket_count = 512) as select * from customer;
Create and populate a partitioned table customers_p to speed up lookups on "city+state" columns:
-- create partitioned table, bucketing on combination of city + state columns -- create table customer_p with (bucketed_on = array['city','state'] , bucket_count=512, max_file_size = '256MB', max_time_range='30d'); create table customer_p with (bucketed_on = array['city','state'] , bucket_count=512); -- update for beta -- Insert new records into the partitioned table insert into customer_p values (.....); insert overwrite into customer_p values (....); -- accelerates queries that test equality on all bucketing columns select * from customer_p where city='San Francisco' and state='California' ; select * from customer_p where city IN ('San Francisco', 'San Jose') and state='California' and monthly_income > 10000; -- NOT accelerated: filter predicate does not use all hash columns select * from customer_p where state='California'; select * from customer_p where city IN ('San Francisco', 'San Jose');
UDP Advanced Use Case Details
Choosing Bucket Count, Partition Size in storage and Time Ranges for Partitions
Bucket counts must be powers of two. Higher bucket count means dividing data among many smaller partitions, which can be less efficient to scan. TD suggests starting with 512 for most cases. If you aren't sure of the best bucket count, it is safer to err on the low side.
We recommend partitioning UDP tables on one-day or multiple-day time ranges, instead of the one-hour partitions most commonly used in TD. Otherwise you can wind up with higher costs and slower data access because too many small partitions have to be fetched from storage.
Aggregations on the hash key
Using a GROUP BY key as the bucketing key, major improvements in performance and reduction in cluster load on aggregation queries were seen. For example, you can see the UDP version of this query on a 1TB table:
- used 10 Presto workers instead of 19
- ran in 45 seconds instead of 2 minutes 31 seconds
processing >3x as many rows per second. (The total data processed in GB was greater because the UDP version of the table occupied more storage.)
presto:udp_tpch_sf1000> select count(*) from (select max_by(l_discount, time), max_by(l_partkey, time) from lineitem group by l_orderkey); _col0 ------------ 1500000000 (1 row) Query 20171227_014452_14154_sjh9g, FINISHED, 10 nodes Splits: 517 total, 517 done (100.00%) 0:45 [6B rows, 25.5GB] [134M rows/s, 585MB/s]
presto:udp_tpch_sf1000> select count(*) from (select max_by(l_discount, time), max_by(l_partkey, time) from tpch_sf1000.lineitem group by l_orderkey); _col0 ------------ 1500000000 (1 row) Query 20171227_014549_14273_sjh9g, FINISHED, 19 nodes Splits: 175 total, 175 done (100.00%) 2:31 [6B rows, 18.3GB] [39.7M rows/s, 124MB/s]
Needle-in-a-Haystack Lookup on the Hash Key
The largest improvements – 5x, 10x or more – will be on lookup or filter operations where the partition key column are tested for equality. Only partitions in the bucket from hashing the partition keys are scanned.
For example, consider two tables:
customersis bucketed on
contactsis bucketed on
These queries will improve:
SELECT... FROM customers WHERE customer_id = 10001;
Here with UDP Presto scans only one bucket (the one that 10001 hashes to) if customer_id is the only bucketing key.
SELECT... FROM contacts WHERE country_code='1' and area_code = '650'
and phone like'555-____';
Here with UDP Presto scans only the bucket that matches the hash of country_code 1 + area_code 650.
These queries will not:
SELECT... FROM customers WHERE customer_id >= 10001;
Here UDP will not improve performance, because the predicate doesn't use '='.
SELECT... FROM contacts WHERE area_code = '650' ;
Here UDP will not improve performance, because the predicate does not include both bucketing keys.
Very Large Join Operations
Very large join operations can sometimes run out of memory. Such joins can benefit from UDP. Distributed and colocated joins will use less memory, CPU, and shuffle less data among Presto workers. This may enable you to finish queries that would otherwise run out of resources. To leverage these benefits, you must:
- Make sure the two tables to be joined are partitioned on the same keys
- Use equi-joins across all the partitioning keys
- Set the following options on your join using a magic comment:
-- set session distributed_join = 'true'
-- set session colocated_join = 'true'
Tip: Improving Performance on Skewed Data
When processing a UDP query, Presto ordinarily creates one split of filtering work per bucket (typically 512 splits, for 512 buckets). But if data is not evenly distributed, filtering on skewed bucket could make performance worse -- one Presto worker node will handle the filtering of that skewed set of partitions, and the whole query lags.
To enable higher scan parallelism you can use:
-- set session distributed_bucket='true|false'
When set to true, multiple splits are used to scan the files in a bucket in parallel, increasing performance. The tradeoff is that colocated join is always disabled when distributed_bucket is true. As a result, some operations such as GROUP BY will require shuffling and more memory during execution.
This query hint is most effective with needle-in-a-haystack queries. Even if these queries perform well with the query hint, Customers should test performance with and without the query hint in other use cases on those tables to find the best performance tradeoffs.
The max number of Partitioning keys is less than or equal to 3 keys
If the limit is exceeded, Presto causes the following error message.
'bucketed_on' must be less than 4 columns