Обсуждение: How to convert a double value to a numeric datum type in pgsql?
Hi all,
I'm talking about the source code of pgsql and I want to know how the typing system works in pgsql.
A few functions can help us do the type conversion, for example:
Int32GetDatum -- convert a integer to a datum
Float8GetDatum -- convert double to a datum
cstring_to_text -- convert a string to a text
...
but there are a lot of types in pgsql,how to choose the corresponding function? or is there a more automatic way to do the type conversion(raw chars to a datum, suppose I have the corresponding Form_pg_type instance)?
I ask this question because I don't know how to convert a double value to a numeric datum(the field's type is numeric(10,2)), pg_type shows that numeric's typlen is -1 whose length is variable thus Float8GetDatum is not working..
hello
you have to look to postgresql/src/backend/utils/adt/numeric.c functions - Pavel
2014-02-24 7:45 GMT+01:00 Felix.徐 <ygnhzeus@gmail.com>:
Hi all,I'm talking about the source code of pgsql and I want to know how the typing system works in pgsql.A few functions can help us do the type conversion, for example:Int32GetDatum -- convert a integer to a datumFloat8GetDatum -- convert double to a datumcstring_to_text -- convert a string to a text...but there are a lot of types in pgsql,how to choose the corresponding function? or is there a more automatic way to do the type conversion(raw chars to a datum, suppose I have the corresponding Form_pg_type instance)?I ask this question because I don't know how to convert a double value to a numeric datum(the field's type is numeric(10,2)), pg_type shows that numeric's typlen is -1 whose length is variable thus Float8GetDatum is not working..
2014-02-24 8:42 GMT+01:00 Felix.徐 <ygnhzeus@gmail.com>:
Thanks , I find another function called OidInputFunctionCall which can automatically generate a datum from a string for any data type, but seems it is not recommended in the comments.
it is little bit different - and it needs a second conversion - double -> string
Regards
Pavel
Pavel
2014-02-24 15:32 GMT+08:00 Pavel Stehule <pavel.stehule@gmail.com>:regardsNumeric result = DatumGetNumeric(DirectFunctionCall1(float8_numeric, Float8GetDatum(dx));then you can call float8_numerichelloyou have to look to postgresql/src/backend/utils/adt/numeric.c functions -
Pavel2014-02-24 7:45 GMT+01:00 Felix.徐 <ygnhzeus@gmail.com>:Hi all,I'm talking about the source code of pgsql and I want to know how the typing system works in pgsql.A few functions can help us do the type conversion, for example:Int32GetDatum -- convert a integer to a datumFloat8GetDatum -- convert double to a datumcstring_to_text -- convert a string to a text...but there are a lot of types in pgsql,how to choose the corresponding function? or is there a more automatic way to do the type conversion(raw chars to a datum, suppose I have the corresponding Form_pg_type instance)?I ask this question because I don't know how to convert a double value to a numeric datum(the field's type is numeric(10,2)), pg_type shows that numeric's typlen is -1 whose length is variable thus Float8GetDatum is not working..