Presto and Athena support reading from external tables using a manifest file, which is a text file containing the list of data files to read for querying a table. When an external table is defined in the Hive metastore using manifest files, Presto and Athena can use the list of files in the manifest rather than finding the files by directory listing. This article describes how to set up a Presto and Athena to Delta Lake integration using manifest files and query Delta tables.
You set up a Presto or Athena to Delta Lake integration using the following steps.
Using Spark configured with Delta Lake, run any of the following commands on a Delta table at location
GENERATE symlink_format_manifest FOR TABLE delta.`<path-to-delta-table>`
val deltaTable = DeltaTable.forPath(<path-to-delta-table>) deltaTable.generate("symlink_format_manifest")
DeltaTable deltaTable = DeltaTable.forPath(<path-to-delta-table>); deltaTable.generate("symlink_format_manifest");
deltaTable = DeltaTable.forPath(<path-to-delta-table>) deltaTable.generate("symlink_format_manifest")
See Generate a manifest file for details.
generate command generates manifest files at
<path-to-delta-table>/_symlink_format_manifest/. In other words, the files in this directory will contain the names of the data files (that is, Parquet files) that should be read for reading a snapshot of the Delta table.
Databricks recommends that you define the Delta table in a location that Presto or Athena read directly.
Define a new table in the Hive metastore connected to Presto or Athena using the format
SymlinkTextInputFormatand the manifest location
CREATE EXTERNAL TABLE mytable ([(col_name1 col_datatype1, ...)]) [PARTITIONED BY (col_name2 col_datatype2, ...)] ROW FORMAT SERDE 'org.apache.hadoop.hive.ql.io.parquet.serde.ParquetHiveSerDe' STORED AS INPUTFORMAT 'org.apache.hadoop.hive.ql.io.SymlinkTextInputFormat' OUTPUTFORMAT 'org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat' LOCATION '<path-to-delta-table>/_symlink_format_manifest/' -- location of the generated manifest
SymlinkTextInputFormatconfigures Presto or Athena to compute file splits for
mytableby reading the manifest file instead of using a directory listing to find data files. Replace
mytablewith the name of the external table and
<path-to-delta-table>with the absolute path to the Delta table.
mytablemust be the same schema and have the same partitions as the Delta table.
- The set of
PARTITIONED BYcolumns must be distinct from the set of non-partitioned columns. Furthermore, you cannot specify partitioned columns with
- You cannot use this table definition in Apache Spark; it can be used only by Presto and Athena.
The tool you use to run the command depends on whether Apache Spark and Presto or Athena use the same Hive metastore.
- Same metastore: If both Apache Spark and Presto or Athena use the same Hive metastore, you can define the table using Apache Spark.
- Different metastores: If Apache Spark and Presto or Athena use different metastores, you must define the table using other tools.
- Athena: You can define the external table in Athena.
- Presto: Presto does not support the syntax
CREATE EXTERNAL TABLE ... STORED AS ..., so you must use another tool (for example, Spark or Hive) connected to the same metastore as Presto to create the table.
If the Delta table is partitioned, run
MSCK REPAIR TABLE mytableafter generating the manifests to force the metastore (connected to Presto or Athena) to discover the partitions. This is needed because the manifest of a partitioned table is itself partitioned in the same directory structure as the table. Run this command using the same tool used to create the table. Furthermore, you should run this command:
- After every manifest generation: New partitions are likely to be visible immediately after the manifest files have been updated. However, doing this too frequently can cause high load for the Hive metastore.
- As frequently as new partitions are expected: For example, if a table is partitioned by date, then you can run repair once after every midnight, after the new partition has been created in the table and its corresponding manifest files have been generated.
When the data in a Delta table is updated you must regenerate the manifests using either of the following approaches:
Update explicitly: After all the data updates, you can run the
generateoperation to update the manifests.
Update automatically: You can configure a Delta table so that all write operations on the table automatically update the manifests. To enable this automatic mode, set the corresponding table property using the following SQL command.
ALTER TABLE delta.`<path-to-delta-table>` SET TBLPROPERTIES(delta.compatibility.symlinkFormatManifest.enabled=true)
To disable this automatic mode, set this property to
false. In addition, for partitioned tables, you have to run
MSCK REPAIRto ensure the metastore connected to Presto or Athena to update partitions.
After enabling automatic mode on a partitioned table, each write operation updates only manifests corresponding to the partitions that operation wrote to. This incremental update ensures that the overhead of manifest generation is low for write operations. However, this also means that if the manifests in other partitions are stale, enabling automatic mode will not automatically fix it. Therefore, Databricks recommends that you explicitly run
GENERATEto update manifests for the entire table immediately after enabling automatic mode.
Whether to update automatically or explicitly depends on the concurrent nature of write operations on the Delta table and the desired data consistency. For example, if automatic mode is enabled, concurrent write operations lead to concurrent overwrites to the manifest files. With such unordered writes, the manifest files are not guaranteed to point to the latest version of the table after the write operations complete. Hence, if concurrent writes are expected and you want to avoid stale manifests, you should consider explicitly updating the manifest after the expected write operations have completed.
The Presto and Athena integration has known limitations in its behavior.
Whenever Delta Lake generates updated manifests, it atomically overwrites existing manifest files. Therefore, Presto and Athena will always see a consistent view of the data files; it will see all of the old version files or all of the new version files. However, the granularity of the consistency guarantees depends on whether or not the table is partitioned.
- Unpartitioned tables: All the files names are written in one manifest file which is updated atomically. In this case Presto and Athena will see full table snapshot consistency.
- Partitioned tables: A manifest file is partitioned in the same Hive-partitioning-style directory structure as the original Delta table. This means that each partition is updated atomically, and Presto or Athena will see a consistent view of each partition but not a consistent view across partitions. Furthermore, since all manifests of all partitions cannot be updated together, concurrent attempts to generate manifests can lead to different partitions having manifests of different versions. While this consistency guarantee under data change is weaker than that of reading Delta tables with Spark, it is still stronger than formats like Parquet as they do not provide partition-level consistency.
Depending on what storage system you are using for Delta tables, it is possible to get incorrect results when Presto or Athena concurrently queries the manifest while the manifest files are being rewritten. In file system implementations that lack atomic file overwrites, a manifest file may be momentarily unavailable. Hence, use manifests with caution if their updates are likely to coincide with queries from Presto or Athena.
Very large numbers of files can hurt the performance of Presto and Athena. Hence Databricks recommends that you compact the files of the table before generating the manifests. The number of files should not exceed 1000 (for the entire unpartitioned table or for each partition in a partitioned table).
Delta Lake supports schema evolution and queries on a Delta table automatically use the latest schema regardless of the schema defined in the table in the Hive metastore. However, Presto or Athena uses the schema defined in the Hive metastore and will not query with the updated schema until the table used by Presto or Athena is redefined to have the updated schema.
Athena does not support reading manifests from CSE-KMS encrypted tables. See the AWS documentation for the latest information.