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

[毛毛]

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!