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SQOOP-3418: Document decimal support in Hive external import into parquet files
(Fero Szabo via Szabolcs Vasas)
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Szabolcs Vasas
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b94a0bd948
@ -112,10 +112,14 @@ configuring a new Hive table with the correct InputFormat. This feature
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currently requires that all partitions of a table be compressed with the lzop
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codec.
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The user can specify the +\--external-table-dir+ option in the sqoop command to
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work with an external Hive table (instead of a managed table, i.e. the default behavior).
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To import data into an external table, one has to specify +\--hive-import+ in the command
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line arguments. Table creation is also supported with the use of +\--create-hive-table+.
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External table import
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+++++++++++++++++++++
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You can specify the +\--external-table-dir+ option in the sqoop command to
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work with an external Hive table (instead of a managed table, i.e. the default
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behavior). To import data into an external table, one has to specify the
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+\--hive-import+ option in the command line arguments. Table creation is
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also supported with the use of the +\--create-hive-table+ option.
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Importing into an external Hive table:
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----
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@ -126,3 +130,38 @@ Create an external Hive table:
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----
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$ sqoop import --hive-import --create-hive-table --connect $CONN --table $TABLENAME --username $USER --password $PASS --external-table-dir /tmp/foobar_example --hive-table foobar
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----
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Decimals in Hive import using parquet file
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++++++++++++++++++++++++++++++++++++++++++
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As mentioned above, a Hive import is a two-step process in Sqoop:
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first, the data is imported onto HDFS, then a HQL statement is generated and
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executed to create the Hive table.
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During the first step, an Avro schema is generated from the SQL data types.
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This schema is then used in a regular Parquet import. After the data was
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imported onto HDFS successfully, Sqoop takes the Avro schema, maps the Avro
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types to Hive types and to generates the HQL statement to create the table.
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Decimal SQL types are converted to Strings in a parquet import per default,
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so Decimal columns appear as String columns in Hive per default. You can change
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this behavior by enabling logical types for parquet, so that Decimals will be
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properly mapped to the Hive type Decimal as well. This can be done with the
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+sqoop.parquet.logical_types.decimal.enable+ property. As noted in the section
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discussing 'Enabling Logical Types in Avro and Parquet import for numbers',
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you should also specify the default precision and scale and enable padding.
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A limitation of Hive is that the maximum precision and scale is 38. When
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converting to the Hive Decimal type, precision and scale will be reduced
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if necessary to meet this limitation, automatically. The data itself however,
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will only have to adhere to the limitations of the Avro schema, thus values
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with a precision and scale bigger than 38 are allowed and will be present on
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storage, but they won't be readable by Hive, (since Hive is a
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schema-on-read tool).
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Enabling padding and specifying a default precision and scale in a Hive Import:
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----
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$ sqoop import -Dsqoop.avro.decimal_padding.enable=true -Dsqoop.parquet.logical_types.decimal.enable=true
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-Dsqoop.avro.logical_types.decimal.default.precision=38 -Dsqoop.avro.logical_types.decimal.default.scale=10
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--hive-import --connect $CONN --table $TABLENAME --username $USER --password $PASS --as-parquetfile
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----
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@ -472,36 +472,48 @@ Enabling Logical Types in Avro and Parquet import for numbers
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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To enable the use of logical types in Sqoop's avro schema generation,
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i.e. used during both avro and parquet imports, one has to use the
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sqoop.avro.logical_types.decimal.enable flag. This is necessary if one
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i.e. used both during avro and parquet imports, one has to use the
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+sqoop.avro.logical_types.decimal.enable+ property. This is necessary if one
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wants to store values as decimals in the avro file format.
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Padding number types in avro import
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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In case of a parquet import, one has to use the
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+sqoop.parquet.logical_types.decimal.enable+ property.
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Padding number types in avro and parquet import
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+++++++++++++++++++++++++++++++++++++++++++++++
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Certain databases, such as Oracle and Postgres store number and decimal
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values without padding. For example 1.5 in a column declared
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as NUMBER (20,5) is stored as is in Oracle, while the equivalent
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as NUMBER (20, 5) is stored as is in Oracle, while the equivalent
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DECIMAL (20, 5) is stored as 1.50000 in an SQL server instance.
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This leads to a scale mismatch during avro import.
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This leads to a scale mismatch during the import.
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To avoid this error, one can use the sqoop.avro.decimal_padding.enable flag
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to turn on padding with 0s. This flag has to be used together with the
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sqoop.avro.logical_types.decimal.enable flag set to true.
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To avoid this error, one can use the +sqoop.avro.decimal_padding.enable+
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property to turn on padding with 0s during import. Naturally, this property is
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used together with logical types enabled, either in avro or in parquet import.
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Default precision and scale in avro import
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Default precision and scale in avro and parquet import
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++++++++++++++++++++++++++++++++++++++++++++++++++++++
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All of the databases allow users to specify numeric columns without
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a precision or scale. While MS SQL and MySQL translate these into
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a valid precision and scale values, Oracle and Postgres don't.
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valid precision and scale, Oracle and Postgres don't.
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Therefore, when a table contains NUMBER in a table in Oracle or
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NUMERIC/DECIMAL in Postgres, one can specify a default precision and scale
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to be used in the avro schema by using the +sqoop.avro.logical_types.decimal.default.precision+
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and +sqoop.avro.logical_types.decimal.default.scale+ flags.
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When a table contains a NUMBER column in Oracle or NUMERIC/DECIMAL in
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Postgres, one can specify a default precision and scale to be used in the
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avro schema by using the +sqoop.avro.logical_types.decimal.default.precision+
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and +sqoop.avro.logical_types.decimal.default.scale+ properties.
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Avro padding also has to be enabled, if the values are shorter than
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the specified default scale.
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the specified default scale, together with logical types.
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Even though the name of the properties contain 'avro', the very same properties
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(+sqoop.avro.logical_types.decimal.default.precision+ and
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+sqoop.avro.logical_types.decimal.default.scale+)
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can be used to specify defaults during a parquet import as well.
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The implementation of this logic and the padding is database independent.
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However, our tests cover Oracle, Postgres, MS Sql server and MySQL databases
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only, therefore these are the supported ones.
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Large Objects
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^^^^^^^^^^^^^
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@ -838,20 +850,27 @@ $ sqoop import --connect jdbc:mysql://db.foo.com/corp \
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----
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Enabling logical types in avro import and also turning on padding with 0s:
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----
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$ sqoop import -Dsqoop.avro.decimal_padding.enable=true -Dsqoop.avro.logical_types.decimal.enable=true
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--connect $CON --username $USER --password $PASS --query "select * from table_name where \$CONDITIONS"
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--connect $MYCONN --username $MYUSER --password $MYPASS --query "select * from table_name where \$CONDITIONS"
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--target-dir hdfs://nameservice1//etl/target_path --as-avrodatafile --verbose -m 1
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----
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Enabling logical types in avro import and also turning on padding with 0s, while specifying default precision and scale as well:
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----
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$ sqoop import -Dsqoop.avro.decimal_padding.enable=true -Dsqoop.avro.logical_types.decimal.enable=true
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-Dsqoop.avro.logical_types.decimal.default.precision=38 -Dsqoop.avro.logical_types.decimal.default.scale=10
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--connect $CON --username $USER --password $PASS --query "select * from table_name where \$CONDITIONS"
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--connect $MYCONN --username $MYUSER --password $MYPASS --query "select * from table_name where \$CONDITIONS"
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--target-dir hdfs://nameservice1//etl/target_path --as-avrodatafile --verbose -m 1
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----
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Enabling logical types in parquet import and also turning on padding with 0s, while specifying default precision and scale as well:
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----
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$ sqoop import -Dsqoop.parquet.logical_types.decimal.enable=true -Dsqoop.avro.decimal_padding.enable=true
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-Dsqoop.avro.logical_types.decimal.default.precision=38 -Dsqoop.avro.logical_types.decimal.default.scale=10
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--connect $MYCONN --username $MYUSER --password $MYPASS --query "select * from table_name where \$CONDITIONS"
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--target-dir hdfs://nameservice1//etl/target_path --as-parquetfile --verbose -m 1
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----
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