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    Query partitioned tables

    This document describes some specific considerations for querying partitioned tables in BigQuery.

    For general information on running queries in BigQuery, see Running interactive and batch queries .

    Overview

    If a query uses a qualifying filter on the value of the partitioning column, BigQuery can scan the partitions that match the filter and skip the remaining partitions. This process is called partition pruning .

    Partition pruning is the mechanism BigQuery uses to eliminate unnecessary partitions from the input scan. The pruned partitions are not included when calculating the bytes scanned by the query. In general, partition pruning helps reduce query cost.

    Pruning behaviors vary for the different types of partitioning, so you could see a difference in bytes processed when querying tables that are partitioned differently but are otherwise identical. To estimate how many bytes a query will process, perform a dry run .

    Query a time-unit column-partitioned table

    To prune partitions when you query a time-unit column-partitioned table , include a filter on the partitioning column.

    In the following example, assume that dataset.table is partitioned on the transaction_date column. The example query prunes dates before 2016-01-01 .

     SELECT 
  
 * 
  
 FROM 
  
 dataset 
 . 
 table 
 WHERE 
  
 transaction_date 
  
 >= 
  
 '2016-01-01'

    Query an ingestion-time partitioned table

    Ingestion-time partitioned tables contain a pseudocolumn named _PARTITIONTIME , which is the partitioning column. The value of the column is the UTC ingestion time for each row, truncated to the partition boundary (such as hourly or daily), as a TIMESTAMP value.

    For example, if you append data on April 15, 2021, 08:15:00 UTC, the _PARTITIONTIME column for those rows contains the following values:

    • Hourly partitioned table: TIMESTAMP("2021-04-15 08:00:00")
    • Daily partitioned table: TIMESTAMP("2021-04-15")
    • Monthly partitioned table: TIMESTAMP("2021-04-01")
    • Yearly partitioned table: TIMESTAMP("2021-01-01")

    If the partition granularity is daily, the table also contains a pseudocolumn named _PARTITIONDATE . The value is equal to _PARTITIONTIME truncated to a DATE value.

    Both of these pseudocolumn names are reserved. You can't create a column with either name in any of your tables.

    To prune partitions, filter on either of these columns. For example, the following query scans only the partitions between the dates January 1, 2016 and January 2, 2016:

     SELECT 
  
 column 
 FROM 
  
 dataset 
 . 
 table 
 WHERE 
  
 _PARTITIONTIME 
  
 BETWEEN 
  
 TIMESTAMP 
 ( 
 '2016-01-01' 
 ) 
  
 AND 
  
 TIMESTAMP 
 ( 
 '2016-01-02' 
 )

    To select the _PARTITIONTIME pseudocolumn, you must use an alias. For example, the following query selects _PARTITIONTIME by assigning the alias pt to the pseudocolumn:

     SELECT 
  
 _PARTITIONTIME 
  
 AS 
  
 pt 
 , 
  
 column 
 FROM 
  
 dataset 
 . 
 table

    For daily partitioned tables, you can select the _PARTITIONDATE pseudocolumn in the same way:

     SELECT 
  
 _PARTITIONDATE 
  
 AS 
  
 pd 
 , 
  
 column 
 FROM 
  
 dataset 
 . 
 table

    The _PARTITIONTIME and _PARTITIONDATE pseudocolumns are not returned by a SELECT * statement. You must select them explicitly:

     SELECT 
  
 _PARTITIONTIME 
  
 AS 
  
 pt 
 , 
  
 * 
 FROM 
  
 dataset 
 . 
 table

    Handle time zones in ingestion-time partitioned tables

    The value of _PARTITIONTIME is based on the UTC date when the field is populated. If you want to query data based on a time zone other than UTC, choose one of the following options:

    • Adjust for time zone differences in your SQL queries.
    • Use partition decorators to load data into specific ingestion-time partitions, based on a different time zone than UTC.

    Better performance with pseudocolumns

    To improve query performance, use the _PARTITIONTIME pseudocolumn by itself on the left side of a comparison.

    For example, the following two queries are equivalent. Depending on the table size, the second query might perform better, because it places _PARTITIONTIME by itself on the left side of the > operator. Both queries process the same amount of data.

     -- Might be slower. 
 SELECT 
  
 field1 
 FROM 
  
 dataset 
 . 
 table1 
 WHERE 
  
 TIMESTAMP_ADD 
 ( 
 _PARTITIONTIME 
 , 
  
 INTERVAL 
  
 5 
  
 DAY 
 ) 
  
 > 
  
 TIMESTAMP 
 ( 
 "2016-04-15" 
 ); 
 -- Often performs better. 
 SELECT 
  
 field1 
 FROM 
  
 dataset 
 . 
 table1 
 WHERE 
  
 _PARTITIONTIME 
  
 > 
  
 TIMESTAMP_SUB 
 ( 
 TIMESTAMP 
 ( 
 '2016-04-15' 
 ), 
  
 INTERVAL 
  
 5 
  
 DAY 
 );

    To limit the partitions that are scanned in a query, use a constant expression in your filter. The following query limits which partitions are pruned based on the first filter condition in the WHERE clause. However, the second filter condition doesn't limit the scanned partitions, because it uses table values, which are dynamic.

     SELECT 
  
 column 
 FROM 
  
 dataset 
 . 
 table2 
 WHERE 
  
 -- This filter condition limits the scanned partitions: 
  
 _PARTITIONTIME 
  
 BETWEEN 
  
 TIMESTAMP 
 ( 
 '2017-01-01' 
 ) 
  
 AND 
  
 TIMESTAMP 
 ( 
 '2017-03-01' 
 ) 
  
 -- This one doesn't, because it uses dynamic table values: 
  
 AND 
  
 _PARTITIONTIME 
  
 = 
  
 ( 
 SELECT 
  
 MAX 
 ( 
 timestamp 
 ) 
  
 from 
  
 dataset 
 . 
 table1 
 )

    To limit the partitions scanned, don't include any other columns in a _PARTITIONTIME filter. For example, the following query does not limit the scanned partitions, because field1 is a column in the table.

     -- Scans all partitions of table2. No pruning. 
 SELECT 
  
 field1 
 FROM 
  
 dataset 
 . 
 table2 
 WHERE 
  
 _PARTITIONTIME 
  
 + 
  
 field1 
  
 = 
  
 TIMESTAMP 
 ( 
 '2016-03-28' 
 );

    If you often query a particular range of times, consider creating a view that filters on the _PARTITIONTIME pseudocolumn. For example, the following statement creates a view that includes only the most recent seven days of data from a table named dataset.partitioned_table :

     -- This view provides pruning. 
 CREATE 
  
 VIEW 
  
 dataset 
 . 
 past_week 
  
 AS 
  
 SELECT 
  
 * 
  
 FROM 
  
 dataset 
 . 
 partitioned_table 
  
 WHERE 
  
 _PARTITIONTIME 
  
 BETWEEN 
  
 TIMESTAMP_TRUNC 
 ( 
 TIMESTAMP_SUB 
 ( 
 CURRENT_TIMESTAMP 
 , 
  
 INTERVAL 
  
 7 
  
 * 
  
 24 
  
 HOUR 
 ), 
  
 DAY 
 ) 
  
 AND 
  
 TIMESTAMP_TRUNC 
 ( 
 CURRENT_TIMESTAMP 
 , 
  
 DAY 
 );

    For information about creating views, see Creating views .

    Query an integer-range partitioned table

    To prune partitions when you query an integer-range partitioned table , include a filter on the integer partitioning column.

    In the following example, assume that dataset.table is an integer-range partitioned table with a partitioning specification of customer_id:0:100:10 The example query scans the three partitions that start with 30, 40, and 50.

     SELECT 
  
 * 
  
 FROM 
  
 dataset 
 . 
 table 
 WHERE 
  
 customer_id 
  
 BETWEEN 
  
 30 
  
 AND 
  
 50 
 +-------------+-------+ 
 | 
  
 customer_id 
  
 | 
  
 value 
  
 | 
 +-------------+-------+ 
 | 
  
 40 
  
 | 
  
 41 
  
 | 
 | 
  
 45 
  
 | 
  
 46 
  
 | 
 | 
  
 30 
  
 | 
  
 31 
  
 | 
 | 
  
 35 
  
 | 
  
 36 
  
 | 
 | 
  
 50 
  
 | 
  
 51 
  
 | 
 +-------------+-------+

    Partition pruning is not supported for functions over an integer range partitioned column. For example, the following query scans the entire table.

     SELECT 
  
 * 
  
 FROM 
  
 dataset 
 . 
 table 
 WHERE 
  
 customer_id 
  
 + 
  
 1 
  
 BETWEEN 
  
 30 
  
 AND 
  
 50

    Query data in the write-optimized storage

    The __UNPARTITIONED__ partition temporarily holds data that is streamed to a partitioned table while it is in the write-optimized storage . Data that is streamed directly to a specific partition of a partitioned table does not use the __UNPARTITIONED__ partition. Instead, the data is streamed directly to the partition.

    Data in the write-optimized storage has NULL values in the _PARTITIONTIME and _PARTITIONDATE columns.

    To query data in the __UNPARTITIONED__ partition, use the _PARTITIONTIME pseudocolumn with the NULL value. For example:

     SELECT 
  
 column 
 FROM 
  
 dataset 
 . 
 table 
 WHERE 
  
 _PARTITIONTIME 
  
 IS 
  
 NULL

    For more information, see Streaming into partitioned tables .

    Best practices for partition pruning

    Use a constant filter expression

    To limit the partitions that are scanned in a query, use a constant expression in your filter. If you use dynamic expressions in your query filter, BigQuery must scan all of the partitions.

    For example, the following query prunes partitions because the filter contains a constant expression:

     SELECT 
  
 t1 
 . 
 name 
 , 
  
 t2 
 . 
 category 
 FROM 
  
 table1 
  
 AS 
  
 t1 
 INNER 
  
 JOIN 
  
 table2 
  
 AS 
  
 t2 
 ON 
  
 t1 
 . 
 id_field 
  
 = 
  
 t2 
 . 
 field2 
 WHERE 
  
 t1 
 . 
 ts 
  
 = 
  
 CURRENT_TIMESTAMP 
 ()

    However, the following query doesn't prune partitions, because the filter, WHERE t1.ts = (SELECT timestamp from table where key = 2) , is not a constant expression; it depends on the dynamic values of the timestamp and key fields:

     SELECT 
  
 t1 
 . 
 name 
 , 
  
 t2 
 . 
 category 
 FROM 
  
 table1 
  
 AS 
  
 t1 
 INNER 
  
 JOIN 
  
 table2 
  
 AS 
  
 t2 
 ON 
  
 t1 
 . 
 id_field 
  
 = 
  
 t2 
 . 
 field2 
 WHERE 
  
 t1 
 . 
 ts 
  
 = 
  
 ( 
 SELECT 
  
 timestamp 
  
 from 
  
 table3 
  
 where 
  
 key 
  
 = 
  
 2 
 )

    Isolate the partition column in your filter

    Isolate the partition column when expressing a filter. Filters that require data from multiple fields to compute will not prune partitions. For example, a query with a date comparison using the partitioning column and a second field, or queries containing some field concatenations will not prune partitions.

    For example, the following filter does not prune partitions because it requires a computation based on the partitioning ts field and a second field ts2 :

    WHERE TIMESTAMP_ADD(ts, INTERVAL 6 HOUR) > ts2

    Require a partition filter in queries

    When you create a partitioned table, you can require the use of predicate filters by enabling the Require partition filteroption. When this option is applied, attempts to query the partitioned table without specifying a WHERE clause produce the following error:

    Cannot query over table 'project_id.dataset.table' without a filter that can be used for partition elimination .

    This requirement also applies to queries on views and materialized views that reference the partitioned table.

    There must be at least one predicate that only references a partition column for the filter to be considered eligible for partition elimination. For example, for a table partitioned on column partition_id with an additional column f in its schema, both of the following WHERE clauses satisfy the requirement:

     WHERE partition_id = "20221231"
WHERE partition_id = "20221231" AND f = "20221130"

    However, WHERE (partition_id = "20221231" OR f = "20221130") is not sufficient.

    For ingestion-time partitioned tables, use either the _PARTITIONTIME or _PARTITIONDATE pseudocolumn.

    For more information about adding the Require partition filteroption when you create a partitioned table, see Creating partitioned tables . You can also update this setting on an existing table.

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