CAST keyword

Type conversion. Can be either:

• Explicit via cast()
• Implicit, in which case it will be automatically performed when required by the context.

Syntax

where:

• expression can be a constant, a column, or an expression that evaluates to a value.
• type refers to the desired data type.

cast can be used a part of arithmetic expression as normal

Explicit conversion

Types can be converted from one to another using the cast() function.

Examples

SELECT
cast(3L + 2L AS INT) cast1,
cast(1578506142000000 AS TIMESTAMP) cast2,
cast(1578506142000000 AS TIMESTAMP_NS) cast3,
cast('10.2' AS DOUBLE) cast4,
cast('1' AS INT) cast5;

Explicit casting of an expression to a smaller data type may result in loss of data when the output data type is smaller than the expression.

• Casting a decimal number type (float or double) to an integer number type (long, int, short) will result in decimals drop.
• If the integer part being cast is larger than the resulting data type, it will be resized by truncating bits.
• Conversions from char to a number type will return the corresponding unicode number and vice versa.

Precision loss examples

SELECT
cast(3.5 + 2 AS INT),
cast(7234623 AS SHORT),
cast(2334444.323 AS SHORT);

When casting numbers into a smaller data type, QuestDB will truncate the higher bits of this number.

Implicit conversion

Type casting may be necessary in certain context such as

• Operations involving various different types
• Inserting values where the originating type is different from the destination column type.

QuestDB will attempt to convert to the data type required by the context. This is called implicit cast and does not require using the cast() function.

Implicit casts are only performed when they would NOT:

1. Reduce overall precision
2. Truncate potential results

Implicit casting also prevents data loss.

When an operation involves multiple types, the resulting type will be the smallest possible type so that no data is lost.

Casting table

The below chart illustrates the explicit and implicit cast available in QuestDB:

SELECT
1234L + 567,
1234L + 0.567,
to_timestamp('2019-10-17T00:00:00', 'yyyy-MM-ddTHH:mm:ss') + 323,
to_timestamp('2019-10-17T00:00:00', 'yyyy-MM-ddTHH:mm:ss') + 0.323;

Alternate syntax

There is a shorthand cast syntax.

Using the above example:

SELECT
cast(3.5 + 2 AS INT),
cast(7234623 AS SHORT),
cast(2334444.323 AS SHORT);

We can use the :: syntax to shorten things up:

SELECT
    (3.5 + 2)::INT,
    7234623::SHORT,
    2334444.323::SHORT;

Which to choose?

It's all preference, however many consider the short hand to be more readable.