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LOAD DATA statement

The LOAD DATA statement streamlines the ETL process for an internal Impala table by moving a data file or all the data files in a directory from an HDFS location into the Impala data directory for that table.

Syntax: 

LOAD DATA INPATH 'hdfs_file_or_directory_path' [OVERWRITE] INTO TABLE tablename
  [PARTITION (partcol1=val1, partcol2=val2 ...)]

When the LOAD DATA statement operates on a partitioned table, it always operates on one partition at a time. Specify the PARTITION clauses and list all the partition key columns, with a constant value specified for each.

Statement type: DML (but still affected by the SYNC_DDL query option)

Usage notes:

  • The loaded data files are moved, not copied, into the Impala data directory.
  • You can specify the HDFS path of a single file to be moved, or the HDFS path of a directory to move all the files inside that directory. You cannot specify any sort of wildcard to take only some of the files from a directory. When loading a directory full of data files, keep all the data files at the top level, with no nested directories underneath.
  • Currently, the Impala LOAD DATA statement only imports files from HDFS, not from the local filesystem. It does not support the LOCAL keyword of the Hive LOAD DATA statement. You must specify a path, not an hdfs:// URI.
  • In the interest of speed, only limited error checking is done. If the loaded files have the wrong file format, different columns than the destination table, or other kind of mismatch, Impala does not raise any error for the LOAD DATA statement. Querying the table afterward could produce a runtime error or unexpected results. Currently, the only checking the LOAD DATA statement does is to avoid mixing together uncompressed and LZO-compressed text files in the same table.
  • When you specify an HDFS directory name as the LOAD DATA argument, any hidden files in that directory (files whose names start with a .) are not moved to the Impala data directory.
  • The operation fails if the source directory contains any non-hidden directories. Prior to Impala 2.5 if the source directory contained any subdirectory, even a hidden one such as _impala_insert_staging, the LOAD DATA statement would fail. In Impala 2.5 and higher, LOAD DATA ignores hidden subdirectories in the source directory, and only fails if any of the subdirectories are non-hidden.
  • The loaded data files retain their original names in the new location, unless a name conflicts with an existing data file, in which case the name of the new file is modified slightly to be unique. (The name-mangling is a slight difference from the Hive LOAD DATA statement, which replaces identically named files.)
  • By providing an easy way to transport files from known locations in HDFS into the Impala data directory structure, the LOAD DATA statement lets you avoid memorizing the locations and layout of HDFS directory tree containing the Impala databases and tables. (For a quick way to check the location of the data files for an Impala table, issue the statement DESCRIBE FORMATTED table_name.)
  • The PARTITION clause is especially convenient for ingesting new data for a partitioned table. As you receive new data for a time period, geographic region, or other division that corresponds to one or more partitioning columns, you can load that data straight into the appropriate Impala data directory, which might be nested several levels down if the table is partitioned by multiple columns. When the table is partitioned, you must specify constant values for all the partitioning columns.

Complex type considerations:

Because Impala currently cannot create Parquet data files containing complex types (ARRAY, STRUCT, and MAP), the LOAD DATA statement is especially important when working with tables containing complex type columns. You create the Parquet data files outside Impala, then use either LOAD DATA, an external table, or HDFS-level file operations followed by REFRESH to associate the data files with the corresponding table.

If you connect to different Impala nodes within an impala-shell session for load-balancing purposes, you can enable the SYNC_DDL query option to make each DDL statement wait before returning, until the new or changed metadata has been received by all the Impala nodes.

Examples:

First, we use a trivial Python script to write different numbers of strings (one per line) into files stored in the doc_demo HDFS user account. (Substitute the path for your own HDFS user account when doing hdfs dfs operations like these.) 

random_strings.py 1000 | hdfs dfs -put - /user/doc_demo/thousand_strings.txt
random_strings.py 100 | hdfs dfs -put - /user/doc_demo/hundred_strings.txt
random_strings.py 10 | hdfs dfs -put - /user/doc_demo/ten_strings.txt

Next, we create a table and load an initial set of data into it. Remember, unless you specify a STORED AS clause, Impala tables default to TEXTFILE format with Ctrl-A (hex 01) as the field delimiter. This example uses a single-column table, so the delimiter is not significant. For large-scale ETL jobs, you would typically use binary format data files such as Parquet or Avro, and load them into Impala tables that use the corresponding file format. 

[localhost:21000] > create table t1 (s string);
[localhost:21000] > load data inpath '/user/doc_demo/thousand_strings.txt' into table t1;
Query finished, fetching results ...
+----------------------------------------------------------+
| summary                                                  |
+----------------------------------------------------------+
| Loaded 1 file(s). Total files in destination location: 1 |
+----------------------------------------------------------+
Returned 1 row(s) in 0.61s
[kilo2-202-961.cs1cloud.internal:21000] > select count(*) from t1;
Query finished, fetching results ...
+------+
| _c0  |
+------+
| 1000 |
+------+
Returned 1 row(s) in 0.67s
[localhost:21000] > load data inpath '/user/doc_demo/thousand_strings.txt' into table t1;
ERROR: AnalysisException: INPATH location '/user/doc_demo/thousand_strings.txt' does not exist.

As indicated by the message at the end of the previous example, the data file was moved from its original location. The following example illustrates how the data file was moved into the Impala data directory for the destination table, keeping its original filename: 

$ hdfs dfs -ls /user/hive/warehouse/load_data_testing.db/t1
Found 1 items
-rw-r--r--   1 doc_demo doc_demo      13926 2013-06-26 15:40 /user/hive/warehouse/load_data_testing.db/t1/thousand_strings.txt

The following example demonstrates the difference between the INTO TABLE and OVERWRITE TABLE clauses. The table already contains 1000 rows. After issuing the LOAD DATA statement with the INTO TABLE clause, the table contains 100 more rows, for a total of 1100. After issuing the LOAD DATA statement with the OVERWRITE INTO TABLE clause, the former contents are gone, and now the table only contains the 10 rows from the just-loaded data file. 

[localhost:21000] > load data inpath '/user/doc_demo/hundred_strings.txt' into table t1;
Query finished, fetching results ...
+----------------------------------------------------------+
| summary                                                  |
+----------------------------------------------------------+
| Loaded 1 file(s). Total files in destination location: 2 |
+----------------------------------------------------------+
Returned 1 row(s) in 0.24s
[localhost:21000] > select count(*) from t1;
Query finished, fetching results ...
+------+
| _c0  |
+------+
| 1100 |
+------+
Returned 1 row(s) in 0.55s
[localhost:21000] > load data inpath '/user/doc_demo/ten_strings.txt' overwrite into table t1;
Query finished, fetching results ...
+----------------------------------------------------------+
| summary                                                  |
+----------------------------------------------------------+
| Loaded 1 file(s). Total files in destination location: 1 |
+----------------------------------------------------------+
Returned 1 row(s) in 0.26s
[localhost:21000] > select count(*) from t1;
Query finished, fetching results ...
+-----+
| _c0 |
+-----+
| 10  |
+-----+
Returned 1 row(s) in 0.62s

Amazon S3 considerations:

In Impala 2.6 and higher, the Impala DML statements (INSERT, LOAD DATA, and CREATE TABLE AS SELECT) can write data into a table or partition that resides in the Amazon Simple Storage Service (S3). The syntax of the DML statements is the same as for any other tables, because the S3 location for tables and partitions is specified by an s3a:// prefix in the LOCATION attribute of CREATE TABLE or ALTER TABLE statements. If you bring data into S3 using the normal S3 transfer mechanisms instead of Impala DML statements, issue a REFRESH statement for the table before using Impala to query the S3 data.

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 LOAD DATA statement and the final stage of the INSERT and CREATE TABLE AS SELECT statements involve moving files from one directory to another. (In the case of INSERT and CREATE TABLE AS SELECT, the files are moved from a temporary staging directory to the final destination directory.) Because S3 does not support a rename operation for existing objects, in these cases Impala actually copies the data files from one location to another and then removes the original files. In Impala 2.6, the S3_SKIP_INSERT_STAGING query option provides a way to speed up INSERT statements for S3 tables and partitions, with the tradeoff that a problem during statement execution could leave data in an inconsistent state. It does not apply to INSERT OVERWRITE or LOAD DATA statements.

ADLS considerations:

In Impala 2.9 and higher, the Impala DML statements (INSERT, LOAD DATA, and CREATE TABLE AS SELECT) can write data into a table or partition that resides in the Azure Data Lake Store (ADLS). ADLS Gen2 is supported in Impala 3.1 and higher.

In theCREATE TABLE or ALTER TABLE statements, specify the ADLS location for tables and partitions with the adl:// prefix for ADLS Gen1 and abfs:// or abfss:// for ADLS Gen2 in the LOCATION attribute.

If you bring data into ADLS using the normal ADLS transfer mechanisms instead of Impala DML statements, issue a REFRESH statement for the table before using Impala to query the ADLS data.

Cancellation: Cannot be cancelled.

HDFS permissions:

The user ID that the impalad daemon runs under, typically the impala user, must have read and write permissions for the files in the source directory, and write permission for the destination directory.

Kudu considerations:

The LOAD DATA statement cannot be used with Kudu tables.

HBase considerations:

The LOAD DATA statement cannot be used with HBase tables.

Related information:

The LOAD DATA statement is an alternative to the INSERT statement. Use LOAD DATA when you have the data files in HDFS but outside of any Impala table.

The LOAD DATA statement is also an alternative to the CREATE EXTERNAL TABLE statement. Use LOAD DATA when it is appropriate to move the data files under Impala control rather than querying them from their original location.