DECIMAL Data Type (Impala 3.0 or higher only)

The DECIMAL data type is a numeric data type with fixed scale and precision.

The data type is useful for storing and doing operations on precise decimal values.

Syntax:

DECIMAL[(precision[, scale])]

Precision:

precision represents the total number of digits that can be represented regardless of the location of the decimal point.

This value must be between 1 and 38, specified as an integer literal.

The default precision is 9.

Scale:

scale represents the number of fractional digits.

This value must be less than or equal to the precision, specified as an integer literal.

The default scale is 0.

When the precision and the scale are omitted, a DECIMAL is treated as DECIMAL(9, 0).

Range:

The range of DECIMAL type is -10^38 +1 through 10^38 –1.

The largest value is represented by DECIMAL(38, 0).

The most precise fractional value (between 0 and 1, or 0 and -1) is represented by DECIMAL(38, 38), with 38 digits to the right of the decimal point. The value closest to 0 would be .0000…1 (37 zeros and the final 1). The value closest to 1 would be .999… (9 repeated 38 times).

Memory and disk storage:

Only the precision determines the storage size for DECIMAL values, and the scale setting has no effect on the storage size. The following table describes the in-memory storage once the values are loaded into memory.

The on-disk representation varies depending on the file format of the table.

Text, RCFile, and SequenceFile tables use ASCII-based formats as below:

• Leading zeros are not stored.
• Trailing zeros are stored.

• Each DECIMAL value takes up as many bytes as the precision of the value, plus:

  • One extra byte if the decimal point is present.
  • One extra byte for negative values.

Parquet and Avro tables use binary formats and offer more compact storage for DECIMAL values. In these tables, Impala stores each value in fewer bytes where possible depending on the precision specified for the DECIMAL column. To conserve space in large tables, use the smallest-precision DECIMAL type.

Precision and scale in arithmetic operations:

For all arithmetic operations, the resulting precision is at most 38.

If the resulting precision would be greater than 38, Impala truncates the result from the back, but keeps at least 6 fractional digits in scale and rounds.

For example, DECIMAL(38, 20) * DECIMAL(38, 20) returns DECIMAL(38, 6). According to the table below, the resulting precision and scale would be (77, 40), but they are higher than the maximum precision and scale for DECIMAL. So, Impala sets the precision to the maximum allowed 38, and truncates the scale to 6.

When you use DECIMAL values in arithmetic operations, the precision and scale of the result value are determined as follows. For better readability, the following terms are used in the table below:

• P1, P2: Input precisions

• S1, S2: Input scales

• L1, L2: Leading digits in input DECIMALs, i.e., L1 = P1 - S1 and L2 = P2 - S2

Precision and scale in functions:

When you use DECIMAL values in built-in functions, the precision and scale of the result value are determined as follows:

• The result of the SUM aggregate function on a DECIMAL value is:

  • Precision: 38

  • Scale: The same scale as the input column

• The result of AVG aggregate function on a DECIMAL value is:

  • Precision: 38

  • Scale: max(Scale of input column, 6)

Implicit conversions in DECIMAL assignments:

Impala enforces strict conversion rules in decimal assignments like in INSERT and UNION statements, or in functions like COALESCE.

If there is not enough precision and scale in the destination, Impala fails with an error.

Impala performs implicit conversions between DECIMAL and other numeric types as below:

• DECIMAL is implicitly converted to DOUBLE or FLOAT when necessary even with a loss of precision. It can be necessary, for example when inserting a DECIMAL value into a DOUBLE column. For example:

  CREATE TABLE flt(c FLOAT);
  INSERT INTO flt SELECT CAST(1e37 AS DECIMAL(38, 0));
  SELECT CAST(c AS DECIMAL(38, 0)) FROM flt;
  Result: 9999999933815812510711506376257961984

  The result has a loss of information due to implicit casting. This is why we discourage using the DOUBLE and FLOAT types in general.

• DOUBLE and FLOAT cannot be implicitly converted to DECIMAL. An error is returned.

• DECIMAL is implicitly converted to DECIMAL if all digits fit in the resulting DECIMAL.

  For example, the following query returns an error because the resulting type that guarantees that all digits fit cannot be determined .

  SELECT GREATEST (CAST(1 AS DECIMAL(38, 0)), CAST(2 AS DECIMAL(38, 37)));

• Integer values can be implicitly converted to DECIMAL when there is enough room in the DECIMAL to guarantee that all digits fit. The integer types require the following numbers of digits to the left of the decimal point when converted to DECIMAL:

  • BIGINT: 19 digits

  • INT: 10 digits

  • SMALLINT: 5 digits

  • TINYINT: 3 digits

  For example:

  CREATE TABLE decimals_10_8 (x DECIMAL(10, 8));
  INSERT INTO decimals_10_8 VALUES (CAST(1 AS TINYINT));

  The above INSERT statement fails because TINYINT requires room for 3 digits to the left of the decimal point in the DECIMAL.

  CREATE TABLE decimals_11_8(x DECIMAL(11, 8));
  INSERT INTO decimals_11_8 VALUES (CAST(1 AS TINYINT));

  The above INSERT statement succeeds because there is enough room for 3 digits to the left of the decimal point that TINYINT requires.

In UNION, the resulting precision and scales are determined as follows.

• Precision: max (L1, L2) + max (S1, S2)

  If the resulting type does not fit in the DECIMAL type, an error is returned. See the first example below.

• Scale: max (S1, S2)

Examples for UNION:

• DECIMAL(20, 0) UNION DECIMAL(20, 20) would require a DECIMAL(40, 20) to fit all the digits. Since this is larger than the max precision for DECIMAL, Impala returns an error. One way to fix the error is to cast both operands to the desired type, for example DECIMAL(38, 18).

• DECIMAL(20, 2) UNION DECIMAL(8, 6) returns DECIMAL(24, 6).

• INT UNION DECIMAL(9, 4) returns DECIMAL(14, 4).

  INT has the precision 10 and the scale 0, so it is treated as DECIMAL(10, 0) UNION DECIMAL(9. 4).

Casting between DECIMAL and other data types:

To avoid potential conversion errors, use CAST to explicitly convert between DECIMAL and other types in decimal assignments like in INSERT and UNION statements, or in functions like COALESCE:

• You can cast the following types to DECIMAL: FLOAT, TINYINT, SMALLINT, INT, BIGINT, STRING

• You can cast DECIMAL to the following types: FLOAT, TINYINT, SMALLINT, INT, BIGINT, STRING, BOOLEAN, TIMESTAMP

Impala performs CAST between DECIMAL and other numeric types as below:

• Precision: If you cast a value with bigger precision than the precision of the destination type, Impala returns an error. For example, CAST(123456 AS DECIMAL(3,0)) returns an error because all digits do not fit into DECIMAL(3, 0)

• Scale: If you cast a value with more fractional digits than the scale of the destination type, the fractional digits are rounded. For example, CAST(1.239 AS DECIMAL(3, 2)) returns 1.24.

Casting STRING to DECIMAL:

You can cast STRING of numeric characters in columns, literals, or expressions to DECIMAL as long as number fits within the specified target DECIMAL type without overflow.

• If scale in STRING > scale in DECIMAL, the fractional digits are rounded to the DECIMAL scale.

  For example, CAST('98.678912' AS DECIMAL(15, 1)) returns 98.7.

• If # leading digits in STRING > # leading digits in DECIMAL, an error is returned.

  For example, CAST('123.45' AS DECIMAL(2, 2)) returns an error.

Exponential notation is supported when casting from STRING.

For example, CAST('1.0e6' AS DECIMAL(32, 0)) returns 1000000.

Casting any non-numeric value, such as 'ABC' to the DECIMAL type returns an error.

Casting DECIMAL to TIMESTAMP:

Casting a DECIMAL value N to TIMESTAMP produces a value that is N seconds past the start of the epoch date (January 1, 1970).

DECIMAL vs FLOAT consideration:

The FLOAT and DOUBLE types can cause problems or unexpected behavior due to inability to precisely represent certain fractional values, for example dollar and cents values for currency. You might find output values slightly different than you inserted, equality tests that do not match precisely, or unexpected values for GROUP BY columns. The DECIMAL type can help reduce unexpected behavior and rounding errors, but at the expense of some performance overhead for assignments and comparisons.

Literals and expressions:

• Numeric literals without a decimal point

  • The literals are treated as the smallest integer that would fit the literal. For example, 111 is a TINYINT, and 1111 is a SMALLINT.
  • Large literals that do not fit into any integer type are treated as DECIMAL.
  • The literals too large to fit into a DECIMAL(38, 0) are treated as DOUBLE.

• Numeric literals with a decimal point

  • The literal with less than 38 digits are treated as DECIMAL.
  • The literals with 38 or more digits are treated as a DOUBLE.
• Exponential notation is supported in DECIMAL literals.

• To represent a very large or precise DECIMAL value as a literal, for example one that contains more digits than can be represented by a BIGINT literal, use a quoted string or a floating-point value for the number and CAST the string to the desired DECIMAL type.

  For example: CAST('999999999999999999999999999999' AS DECIMAL(38, 5)))

File format considerations:

The DECIMAL data type can be stored in any of the file formats supported by Impala.

• Impala can query Avro, RCFile, or SequenceFile tables that contain DECIMAL columns, created by other Hadoop components.

• Impala can query and insert into Kudu tables that contain DECIMAL columns.

• The DECIMAL data type is fully compatible with HBase tables.

• The DECIMAL data type is fully compatible with Parquet tables.

• Values of the DECIMAL data type are potentially larger in text tables than in tables using Parquet or other binary formats.

UDF consideration:

When writing a C++ UDF, use the DecimalVal data type defined in /usr/include/impala_udf/udf.h.

Changing precision and scale:

You can issue an ALTER TABLE ... REPLACE COLUMNS statement to change the precision and scale of an existing DECIMAL column.

• For text-based formats (text, RCFile, and SequenceFile tables)

  • If the values in the column fit within the new precision and scale, they are returned correctly by a query.

  • If any values that do not fit within the new precision and scale:

    • Impala returns an error if the query option ABORT_ON_ERROR is set to true.
    • Impala returns a NULL and warning that conversion failed if the query option ABORT_ON_ERROR is set to false.

  • Leading zeros do not count against the precision value, but trailing zeros after the decimal point do.

• For binary formats (Parquet and Avro tables)

  • Although an ALTER TABLE ... REPLACE COLUMNS statement that changes the precision or scale of a DECIMAL column succeeds, any subsequent attempt to query the changed column results in a fatal error. This is because the metadata about the columns is stored in the data files themselves, and ALTER TABLE does not actually make any updates to the data files. The other unaltered columns can still be queried successfully.

  • If the metadata in the data files disagrees with the metadata in the metastore database, Impala cancels the query.

Partitioning:

Using a DECIMAL column as a partition key provides you a better match between the partition key values and the HDFS directory names than using a DOUBLE or FLOAT partitioning column.

Column statistics considerations:

Because the DECIMAL type has a fixed size, the maximum and average size fields are always filled in for column statistics, even before you run the COMPUTE STATS statement.

Compatibility with older version of DECIMAL:

This version of DECIMAL type is the default in Impala 3.0 and higher. The key differences between this version of DECIMAL and the previous DECIMAL V1 in Impala 2.x include the following.

If you need to continue using the first version of the DECIMAL type for the backward compatibility of your queries, set the DECIMAL_V2 query option to FALSE:

SET DECIMAL_V2=FALSE;

Compatibility with other databases:

Use the DECIMAL data type in Impala for applications where you used the NUMBER data type in Oracle.

The Impala DECIMAL type does not support the Oracle idioms of * for scale.

The Impala DECIMAL type does not support negative values for precision.

Parent topic: Data Types