![]() ![]() Notice that the cast syntax with the cast operator (::) is PostgreSQL-specific and does not conform to the SQL standard PostgreSQL CAST examples '0':: DATE Code language: SQL (Structured Query Language) ( sql ) See the following example: SELECT '100':: INTEGER, Then, specify the target data type to which you want to convert the result of the expression.īesides the type CAST syntax, you can use the following syntax to convert a value of one type into another: expression::type Code language: SQL (Structured Query Language) ( sql ). ![]() First, specify an expression that can be a constant, a table column, an expression that evaluates to a value.The following illustrates the syntax of type CAST: CAST ( expression AS target_type ) Code language: SQL (Structured Query Language) ( sql ) PostgreSQL provides you with the CAST operator that allows you to do this. There are many cases that you want to convert a value of one data type into another. Setting a value of 0 restores the previous default of rounding the value to 6 (for float4) or 15 (for float8) significant decimal digits.Summary: in this tutorial, we will show you how to use PostgreSQL CAST operator to convert a value of one type to another. It could be a bug or an oversight, but there could be a genuine reason for that (which I don't know unfortunately).įor compatibility with output generated by older versions of PostgreSQL, and to allow the output precision to be reduced, the extra_float_digits parameter can be used to select rounded decimal output instead. It likely means somewhere in Postgres code it uses "old" precision logic for this conversion. If you increase extra_float_digits (the default value is 1, but you can increase it up to 3), the precision of conversion to float8 changes (but not past 1 digit), but the precision of conversion from float8 to numeric doesn't change. The observed behavior of conversion between float8 and numeric makes sense if we assume that extra_float_digits value is set to 0 SET extra_float_digits = 0 So maybe there are good reasons for capping at 15 digits that I fail to see?īigint will be more precise anyway (19 digits at most). A cast to numeric loses precision, while a cast to bigint does not. This has counter-intuitive (at least for me) effects. So why not preserve at most 17 significant decimal digits in the cast to numeric? The cast to bigint does better! (However, the output value is currently never exactly midway between two representable values, in order to avoid a widespread bug where input routines do not properly respect the round-to-nearest-even rule.) This value will use at most 17 significant decimal digits for float8 values, and at most 9 digits for float4 values. The double precision type has a range of around 1E-307 to 1E+308 with a precision of at least 15 digits.īy default, floating point values are output in text form in their shortest precise decimal representation the decimal value produced is closer to the true stored binary value than to any other value representable in the same binary precision. In the chapter for floating point numbers: Some relevant quotes from the current manual (Postgres 14):ĭouble precision. ![]() I picked values at boundaries where float8 flips a bit - at least in my local installation (Postgres 13, Ubuntu, Intel CPU), and on dbfiddle, and in a hosted DB on AWS, too). The cast to bigint (for values within its range) preserves more precision: SELECT f8 AS float8 The cast from double precision ( float8) to numeric rounds to 15 significant decimal digits, thereby losing information.
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