Ignoring Invalid Data when Casting to Timestamp Type

Casting data from one type to another can be a common operation in SQL, but it’s not always straightforward. In the case of timestamp types, invalid values can cause errors or unexpected results. In this article, we’ll explore how to ignore invalid data when casting to a timestamp type.

Understanding PostgreSQL’s Timestamp Type

PostgreSQL’s timestamp type is a complex data structure that represents dates and times. It has several formats, including the ISO 8601 format YYYY-MM-DDTHH:MM:SS.SSZZ, where YYYY is the year, MM is the month, DD is the day, T is the separator for the time part, HH is the hour in 24-hour format, MM is the minute, SS is the second, and SSZ is the timezone offset.

Checking Validity of Timestamp Values

PostgreSQL provides a function called pg_input_is_valid() that checks whether a given string is valid input for a specified data type. This function returns true if the input is valid and false otherwise.

Using pg_input_is_valid()

In PostgreSQL 16 and above, you can use the pg_input_is_valid() function to check the validity of timestamp values before casting them to the timestamp type.

select 
    timestamp_candidate,
    (timestamp_candidate)::timestamp as casted_timestamp,
    pg_input_is_valid(timestamp_candidate::text, 'timestamp') as is_valid
from your_table;

This query checks whether each timestamp value is valid for the timestamp type and returns a boolean indicating whether it’s valid.

Building Your Own Function

In PostgreSQL 15 and earlier, you can build your own function to check the validity of timestamp values. This function will return true if the input is valid and false otherwise.

create or replace function is_interpretable_as(arg text, arg_type text) 
returns boolean language plpgsql as $$
begin
    execute format('select cast(%L as %s)', arg, arg_type);
    return true;
exception when others then
    return false;
end $$;

This function attempts to cast the input string to the specified type. If successful, it returns true. Otherwise, it returns false.

Handling Invalid Values

When using pg_input_is_valid() or your own custom function, you can handle invalid values in several ways:

• Ignore them: Simply ignore rows with invalid timestamp values and continue processing.
• Replace them with a default value: Replace invalid timestamp values with a default value, such as NULL or the current timestamp.

Example Use Cases

Here are some example use cases for ignoring invalid data when casting to a timestamp type:

-- Ignore rows with invalid timestamps
select 
    *
from your_table
where pg_input_is_valid(timestamp_candidate::text, 'timestamp');

-- Replace invalid timestamps with the current timestamp
select 
    replace(timestamp_candidate, invalid_timestamp, current_timestamp) as casted_timestamp
from your_table;

Conclusion

Ignoring invalid data when casting to a timestamp type is an important consideration in SQL processing. By using PostgreSQL’s built-in functions like pg_input_is_valid() or building your own custom function, you can efficiently handle invalid values and ensure accurate results. Remember to choose the approach that best fits your specific use case and data requirements.

Last modified on 2024-01-15