How can I accurately infer the output of a SELECT statement FROM a table with floating-point data type?

[毛毛 <krave@xxxxxxx>]

Hi,

Do we have a rule by follow which one can accurately info the output of a SELECT statment FROM a table with floating-point data type?

I am working with PostgreSQL 16 and pgAdmin 7.6.

Here is my SQL script:

```

CREATE TABLE TEST (REAL_COLUMN real);

INSERT INTO TEST
VALUES (12345.123456789);

SELECT * FROM TEST;

```

I consulted the following doc and found that the range of real type is 6 decimal digits precision.

https://www.postgresql.org/docs/current/datatype-numeric.html#DATATYPE-NUMERIC

So I thought the output of the SELECT statement should be like: 12345.1 with 6 digits in total.

But it turns out to be 12345.123 with 8 digits in total.

May I know why would this happen?

Do we have a rule I can use to infer the correct output of a floating-point number without running the script?

Thank you for your time and have a great day!