Impala with Amazon S3
You can use Impala to query data residing on the Amazon S3 object store. This capability allows convenient access to a storage system that is remotely managed, accessible from anywhere, and integrated with various cloud-based services.
Impala can query files in any supported file format from S3. The S3 storage location can be for an entire table, or individual partitions in a partitioned table.
Best Practices for Using Impala with S3
The following guidelines summarize the best practices described in the rest of this topic:
- Any reference to an S3 location must be fully qualified when S3 is
not designated as the default storage, for example,
- Set the
fs.s3a.connection.maximumsafety valve setting, to 1500 for impalad.
- Set the
fs.s3a.block.sizesafety valve setting to 134217728 (128 MB in bytes) if most Parquet files queried by Impala were written by Hive or ParquetMR jobs.
- Set the
PARQUET_OBJECT_STORE_SPLIT_SIZEquery option to 268435456 (256 MB in bytes) if most Parquet files queried by Impala were written by Impala.
DROP TABLE .. PURGEis much faster than the default
DROP TABLE. The same applies to
ALTER TABLE ... DROP PARTITION PURGEversus the default
DROP PARTITIONoperation. However, due to the eventually consistent nature of S3, the files for that table or partition could remain for some unbounded time when using
PURGE. The default
DROP TABLE/PARTITIONis slow because Impala copies the files to the S3A trashcan, and Impala waits until all the data is moved.
DROP TABLE/PARTITION .. PURGEis a fast delete operation, and the Impala statement finishes quickly even though the change might not have propagated fully throughout S3.
INSERTstatements are faster than
INSERT OVERWRITEfor S3. The
S3_SKIP_INSERT_STAGINGquery option, which is set to
trueby default, skips the staging step for regular
INSERT OVERWRITE). This makes the operation much faster, but consistency is not guaranteed: if a node fails during execution, the table could end up with inconsistent data. Set this option to
falseif stronger consistency is required, but this setting will make the
For Impala-ACID tables, both
INSERT OVERWRITEtables for S3 are fast, regardless of the setting of
S3_SKIP_INSERT_STAGING. Plus, consistency is guaranteed with ACID tables.
- Enable data cache for remote reads.
- Enable S3Guard in your cluster for data consistency.
- Too many files in a table can make metadata loading and updating
slow in S3. If too many requests are made to S3, S3 has a back-off
mechanism and responds slower than usual.
- If you have many small files due to over-granular partitioning, configure partitions with many megabytes of data so that even a query against a single partition can be parallelized effectively.
- If you have many small files because of many small
INSERTqueries, use bulk
INSERTs so that more data is written to fewer files.
Creating Impala Databases, Tables, and Partitions for Data Stored in S3
To create a table that resides in S3, run the
ALTER TABLE statement with the
ALTER TABLE can set the
property for an individual partition, so that some data in a table
resides in S3 and other data in the same table resides on HDFS.
The syntax for the
LOCATION clause is:
The file system prefix is always
s3a://. Impala does
not support the
For a partitioned table, either specify a separate
LOCATION clause for each new partition, or specify a
LOCATION for the table and set up a directory
structure in S3 to mirror the way Impala partitioned tables are
structured in S3.
You point a nonpartitioned table or an individual partition at S3 by specifying a single directory path in S3, which could be any arbitrary directory. To replicate the structure of an entire Impala partitioned table or database in S3 requires more care, with directories and subdirectories nested and named to match the equivalent directory tree in HDFS. Consider setting up an empty staging area if necessary in HDFS, and recording the complete directory structure so that you can replicate it in S3.
When working with multiple tables with data files stored in S3, you
can create a database with a
pointing to an S3 path. Specify a URL of the form
LOCATION attribute of the database. Any tables
created inside that database automatically create directories underneath
the one specified by the database
The following example creates a table with one partition for the year 2017 resides on HDFS and one partition for the year 2018 resides in S3.
The partition for year 2018 includes a
attribute with an
s3a:// URL, and so refers to data
residing in S3, under a specific path underneath the bucket
CREATE TABLE mostly_on_hdfs (x int) PARTITIONED BY (year INT); ALTER TABLE mostly_on_hdfs ADD PARTITION (year=2017); ALTER TABLE mostly_on_hdfs ADD PARTITION (year=2018) LOCATION 's3a://impala-demo/dir1/dir2/dir3/t1';
The following session creates a database and two partitioned tables residing entirely in S3, one partitioned by a single column and the other partitioned by multiple columns.
- Because a
LOCATIONattribute with an
s3a://URL is specified for the database, the tables inside that database are automatically created in S3 underneath the database directory.
- To see the names of the associated subdirectories, including the partition key values, use an S3 client tool to examine how the directory structure is organized in S3.
CREATE DATABASE db_on_s3 LOCATION 's3a://impala-demo/dir1/dir2/dir3'; CREATE TABLE partitioned_multiple_keys (x INT) PARTITIONED BY (year SMALLINT, month TINYINT, day TINYINT); ALTER TABLE partitioned_multiple_keys ADD PARTITION (year=2015,month=1,day=1); ALTER TABLE partitioned_multiple_keys ADD PARTITION (year=2015,month=1,day=31); !hdfs dfs -ls -R s3a://impala-demo/dir1/dir2/dir3 2015-03-17 13:56:34 0 dir1/dir2/dir3/ 2015-03-17 16:47:13 0 dir1/dir2/dir3/partitioned_multiple_keys/ 2015-03-17 16:47:44 0 dir1/dir2/dir3/partitioned_multiple_keys/year=2015/month=1/day=1/ 2015-03-17 16:47:50 0 dir1/dir2/dir3/partitioned_multiple_keys/year=2015/month=1/day=31/
CREATE DATABASE and
statements create the associated directory paths if they do not already
exist. You can specify multiple levels of directories, and the
CREATE statement creates all appropriate levels,
similar to using
Use the standard S3 file upload methods to put the actual data files into the right locations. You can also put the directory paths and data files in place before creating the associated Impala databases or tables, and Impala automatically uses the data from the appropriate location after the associated databases and tables are created.
ALTER TABLE statement with the
LOCATION clause to switch whether an existing table
or partition points to data in HDFS or S3. For example, if you have an
Impala table or partition pointing to data files in HDFS or S3, and you
later transfer those data files to the other filesystem, use the
ALTER TABLE statement to adjust the
LOCATION attribute of the corresponding table or
partition to reflect that change.
Internal and External Tables Located in S3
Just as with tables located on HDFS storage, you can designate
S3-based tables as either internal (managed by Impala) or external with
CREATE TABLE or
TABLE statement respectively.
When you drop an internal table, the files associated with the table are removed, even if they are in S3 storage. When you drop an external table, the files associated with the table are left alone, and are still available for access by other tools or components.
If the data in S3 is intended to be long-lived and accessed by other
tools in addition to Impala, create any associated S3 tables with the
CREATE EXTERNAL TABLE statement, so that the files
are not deleted from S3 when the table is dropped.
If the data in S3 is only needed for querying by Impala and can be
safely discarded once the Impala workflow is complete, create the
associated S3 tables using the
CREATE TABLE statement,
so that dropping the table also deletes the corresponding data files in
Loading Data into S3 for Impala Queries
If your ETL pipeline involves moving data into S3 and then querying through Impala, you can either use Impala DML statements to create, move, or copy the data, or use the same data loading techniques as you would for non-Impala data.
Using Impala DML Statements for S3 Data:
Impala DML statements (
CREATE TABLE AS SELECT) can write
data into a table or partition that resides in S3.
Because of differences between S3 and
traditional filesystems, DML operations for S3 tables can take longer
than for tables on HDFS. For example, both the
DATA statement and the final stage of the
CREATE TABLE AS SELECT
statements involve moving files from one directory to another. (In the
CREATE TABLE AS
SELECT, the files are moved from a temporary staging
directory to the final destination directory.) Because S3 does not
rename operation for existing objects, in these cases,
Impala copies the data files from one location to another and then
removes the original files.
Manually Loading Data into Impala Tables in S3:
You can use the Amazon-provided methods to bring data files into S3 for querying through Impala.
After you upload data files to a location already mapped to an Impala
table or partition, or if you delete files in S3 from such a location
outside of Impala, issue the
REFRESH statement to make
Impala aware of the new set of data files.
Running and Tuning Impala Queries for Data Stored in S3
Once a table or partition is designated as residing in S3, the
SELECT statement transparently accesses the data
files from the appropriate storage layer.
- Queries against S3 data support all the same file formats as HDFS data.
- Tables can be unpartitioned or partitioned. For partitioned tables,
either manually construct paths in S3 based on partition key values,
ALTER TABLE ... ADD PARTITIONto set up the appropriate paths in S3.
- HDFS and HBase tables can be joined to S3 tables, or S3 tables can be joined with each other.
- Authorization to control access to databases, tables, or columns works the same whether the data is in HDFS or in S3.
- The Catalog Server (catalogd) daemon caches metadata for both HDFS and S3 tables.
- Queries against S3 tables are subject to the same kinds of admission control and resource management as HDFS tables.
- Metadata about S3 tables is stored in the same Metastore database as for HDFS tables.
- You can set up views referring to S3 tables.
SHOW TABLE STATS, and
SHOW COLUMN STATSstatements work for S3 tables.
Query Performance for S3
All else being equal, performance is expected to be lower for queries
running against data in S3 than HDFS. The actual mechanics of the
SELECT statement are different when the data is in
S3. Although the work is still distributed across the DataNodes of the
cluster, Impala might parallelize the work for a distributed query
differently for data on HDFS and S3.
S3 does not have the same block notion as HDFS, so Impala uses heuristics to determine how to split up large S3 files for processing in parallel. Because all hosts can access any S3 data file with equal efficiency, the distribution of work might be different than for HDFS data, where the data blocks are physically read using short-circuit local reads by hosts that contain the appropriate block replicas. Although the I/O to read the S3 data might be spread evenly across the hosts of the cluster, the fact that all data is initially retrieved across the network means that the overall query performance is likely to be lower for S3 data than for HDFS data.
PARQUET_OBJECT_STORE_SPLIT_SIZE query option
to control the Parquet-specific split size. The default value is 256
When optimizing aspects of complex queries such as the join order,
Impala treats tables on HDFS and S3 the same way. Therefore, follow all
the same tuning recommendations for S3 tables as for HDFS ones, such as
COMPUTE STATS statement to help Impala
construct accurate estimates of row counts and cardinality.
In query profile reports, the numbers for
BytesReadRemoteUnexpected are blank because those
metrics come from HDFS. By definition, all the I/O for S3 tables
involves remote reads.
Restrictions on Impala Support for S3
The following restrictions apply when using Impala with S3:
- Impala does not support the old
s3n://filesystem schemes, and it only supports
- Although S3 is often used to store JSON-formatted data, the current Impala support for S3 does not include directly querying JSON data. For Impala queries, use data files in one of the file formats listed in Hadoop File Formats Support. If you have data in JSON format, you can prepare a flattened version of that data for querying by Impala as part of your ETL cycle.
- You cannot use the
ALTER TABLE ... SET CACHEDstatement for tables or partitions that are located in S3.