Partitioning for Impala Tables

Partitioning is typically appropriate for:

• Tables that are very large, where reading the entire data set takes an impractical amount of time.
• Tables that are always or almost always queried with conditions on the partitioning columns. In our example of a table partitioned by year, SELECT COUNT(*) FROM school_records WHERE year = 1985 is efficient, only examining a small fraction of the data; but SELECT COUNT(*) FROM school_records has to process a separate data file for each year, resulting in more overall work than in an unpartitioned table. You would probably not partition this way if you frequently queried the table based on last name, student ID, and so on without testing the year.
• Columns that have reasonable cardinality (number of different values). If a column only has a small number of values, for example Male or Female, you do not gain much efficiency by eliminating only about 50% of the data to read for each query. If a column has only a few rows matching each value, the number of directories to process can become a limiting factor, and the data file in each directory could be too small to take advantage of the Hadoop mechanism for transmitting data in multi-megabyte blocks. For example, you might partition census data by year, store sales data by year and month, and web traffic data by year, month, and day. (Some users with high volumes of incoming data might even partition down to the individual hour and minute.)
• Data that already passes through an extract, transform, and load (ETL) pipeline. The values of the partitioning columns are stripped from the original data files and represented by directory names, so loading data into a partitioned table involves some sort of transformation or preprocessing.

In terms of Impala SQL syntax, partitioning affects these statements:

• CREATE TABLE: you specify a PARTITIONED BY clause when creating the table to identify names and data types of the partitioning columns. These columns are not included in the main list of columns for the table.
• In CDH 5.7 / Impala 2.5 and higher, you can also use the PARTITIONED BY clause in a CREATE TABLE AS SELECT statement. This syntax lets you use a single statement to create a partitioned table, copy data into it, and create new partitions based on the values in the inserted data.
• ALTER TABLE: you can add or drop partitions, to work with different portions of a huge data set. You can designate the HDFS directory that holds the data files for a specific partition. With data partitioned by date values, you might "age out" data that is no longer relevant.
• INSERT: When you insert data into a partitioned table, you identify the partitioning columns. One or more values from each inserted row are not stored in data files, but instead determine the directory where that row value is stored. You can also specify which partition to load a set of data into, with INSERT OVERWRITE statements; you can replace the contents of a specific partition but you cannot append data to a specific partition.

  By default, if an INSERT statement creates any new subdirectories underneath a partitioned table, those subdirectories are assigned default HDFS permissions for the impala user. To make each subdirectory have the same permissions as its parent directory in HDFS, specify the --insert_inherit_permissions startup option for the impalad daemon.

• Although the syntax of the SELECT statement is the same whether or not the table is partitioned, the way queries interact with partitioned tables can have a dramatic impact on performance and scalability. The mechanism that lets queries skip certain partitions during a query is known as partition pruning; see Partition Pruning for Queries for details.
• In Impala 1.4 and later, there is a SHOW PARTITIONS statement that displays information about each partition in a table. See SHOW Statement for details.

Specifying all the partition columns in a SQL statement is called static partitioning, because the statement affects a single predictable partition. For example, you use static partitioning with an ALTER TABLE statement that affects only one partition, or with an INSERT statement that inserts all values into the same partition:

insert into t1 partition(x=10, y='a') select c1 from some_other_table;

When you specify some partition key columns in an INSERT statement, but leave out the values, Impala determines which partition to insert. This technique is called dynamic partitioning:

insert into t1 partition(x, y='b') select c1, c2 from some_other_table;
-- Create new partition if necessary based on variable year, month, and day; insert a single value.
insert into weather partition (year, month, day) select 'cloudy',2014,4,21;
-- Create new partition if necessary for specified year and month but variable day; insert a single value.
insert into weather partition (year=2014, month=04, day) select 'sunny',22;

The more key columns you specify in the PARTITION clause, the fewer columns you need in the SELECT list. The trailing columns in the SELECT list are substituted in order for the partition key columns with no specified value.

The REFRESH statement is typically used with partitioned tables when new data files are loaded into a partition by some non-Impala mechanism, such as a Hive or Spark job. The REFRESH statement makes Impala aware of the new data files so that they can be used in Impala queries. Because partitioned tables typically contain a high volume of data, the REFRESH operation for a full partitioned table can take significant time.

In CDH 5.9 / Impala 2.7 and higher, you can include a PARTITION (partition_spec) clause in the REFRESH statement so that only a single partition is refreshed. For example, REFRESH big_table PARTITION (year=2017, month=9, day=30). The partition spec must include all the partition key columns. See REFRESH Statement for more details and examples of REFRESH syntax and usage.

By default, if an INSERT statement creates any new subdirectories underneath a partitioned table, those subdirectories are assigned default HDFS permissions for the impala user. To make each subdirectory have the same permissions as its parent directory in HDFS, specify the --insert_inherit_permissions startup option for the impalad daemon.

Partition pruning refers to the mechanism where a query can skip reading the data files corresponding to one or more partitions. If you can arrange for queries to prune large numbers of unnecessary partitions from the query execution plan, the queries use fewer resources and are thus proportionally faster and more scalable.

For example, if a table is partitioned by columns YEAR, MONTH, and DAY, then WHERE clauses such as WHERE year = 2013, WHERE year < 2010, or WHERE year BETWEEN 1995 AND 1998 allow Impala to skip the data files in all partitions outside the specified range. Likewise, WHERE year = 2013 AND month BETWEEN 1 AND 3 could prune even more partitions, reading the data files for only a portion of one year.

The columns you choose as the partition keys should be ones that are frequently used to filter query results in important, large-scale queries. Popular examples are some combination of year, month, and day when the data has associated time values, and geographic region when the data is associated with some place.

• For time-based data, split out the separate parts into their own columns, because Impala cannot partition based on a TIMESTAMP column.

• The data type of the partition columns does not have a significant effect on the storage required, because the values from those columns are not stored in the data files, rather they are represented as strings inside HDFS directory names.

• In CDH 5.7 / Impala 2.5 and higher, you can enable the OPTIMIZE_PARTITION_KEY_SCANS query option to speed up queries that only refer to partition key columns, such as SELECT MAX(year). This setting is not enabled by default because the query behavior is slightly different if the table contains partition directories without actual data inside. See OPTIMIZE_PARTITION_KEY_SCANS Query Option (CDH 5.7 or higher only) for details.

• Partitioned tables can contain complex type columns. All the partition key columns must be scalar types.

• Remember that when Impala queries data stored in HDFS, it is most efficient to use multi-megabyte files to take advantage of the HDFS block size. For Parquet tables, the block size (and ideal size of the data files) is 256 MB in Impala 2.0 and later. Therefore, avoid specifying too many partition key columns, which could result in individual partitions containing only small amounts of data. For example, if you receive 1 GB of data per day, you might partition by year, month, and day; while if you receive 5 GB of data per minute, you might partition by year, month, day, hour, and minute. If you have data with a geographic component, you might partition based on postal code if you have many megabytes of data for each postal code, but if not, you might partition by some larger region such as city, state, or country. state

If you frequently run aggregate functions such as MIN(), MAX(), and COUNT(DISTINCT) on partition key columns, consider enabling the OPTIMIZE_PARTITION_KEY_SCANS query option, which optimizes such queries. This feature is available in CDH 5.7 / Impala 2.5 and higher. See OPTIMIZE_PARTITION_KEY_SCANS Query Option (CDH 5.7 or higher only) for the kinds of queries that this option applies to, and slight differences in how partitions are evaluated when this query option is enabled.

Partitioned tables have the flexibility to use different file formats for different partitions. (For background information about the different file formats Impala supports, see How Impala Works with Hadoop File Formats.) For example, if you originally received data in text format, then received new data in RCFile format, and eventually began receiving data in Parquet format, all that data could reside in the same table for queries. You just need to ensure that the table is structured so that the data files that use different file formats reside in separate partitions.

For example, here is how you might switch from text to Parquet data as you receive data for different years:

[localhost:21000] > create table census (name string) partitioned by (year smallint);
[localhost:21000] > alter table census add partition (year=2012); -- Text format;

[localhost:21000] > alter table census add partition (year=2013); -- Text format switches to Parquet before data loaded;
[localhost:21000] > alter table census partition (year=2013) set fileformat parquet;

[localhost:21000] > insert into census partition (year=2012) values ('Smith'),('Jones'),('Lee'),('Singh');
[localhost:21000] > insert into census partition (year=2013) values ('Flores'),('Bogomolov'),('Cooper'),('Appiah');

At this point, the HDFS directory for year=2012 contains a text-format data file, while the HDFS directory for year=2013 contains a Parquet data file. As always, when loading non-trivial data, you would use INSERT ... SELECT or LOAD DATA to import data in large batches, rather than INSERT ... VALUES which produces small files that are inefficient for real-world queries.

For other file types that Impala cannot create natively, you can switch into Hive and issue the ALTER TABLE ... SET FILEFORMAT statements and INSERT or LOAD DATA statements there. After switching back to Impala, issue a REFRESH table_name statement so that Impala recognizes any partitions or new data added through Hive.

You can add, drop, set the expected file format, or set the HDFS location of the data files for individual partitions within an Impala table. See ALTER TABLE Statement for syntax details, and Setting Different File Formats for Partitions for tips on managing tables containing partitions with different file formats.

What happens to the data files when a partition is dropped depends on whether the partitioned table is designated as internal or external. For an internal (managed) table, the data files are deleted. For example, if data in the partitioned table is a copy of raw data files stored elsewhere, you might save disk space by dropping older partitions that are no longer required for reporting, knowing that the original data is still available if needed later. For an external table, the data files are left alone. For example, dropping a partition without deleting the associated files lets Impala consider a smaller set of partitions, improving query efficiency and reducing overhead for DDL operations on the table; if the data is needed again later, you can add the partition again. See Overview of Impala Tables for details and examples.