Oracle ORA_DST_AFFECTED Function: A Simple Guide

The ORA_DST_AFFECTED function in Oracle SQL is a highly specialized diagnostic tool used by Database Administrators (DBAs). This function is not used in regular, day-to-day SQL queries.

Its one and only purpose is to help during a database time zone file upgrade. It "checks" a TIMESTAMP WITH TIME ZONE value to see if it will be impacted by a change in Daylight Saving Time (DST) rules.

What is the ORA_DST_AFFECTED Function in Oracle?

When Oracle updates its time zone files (e.g., to DSTv38, DSTv39, etc.), the rules for when Daylight Saving Time begins and ends can change. This can cause some existing TIMESTAMP WITH TIME ZONE data to become invalid.

For example, a new rule might mean a specific recorded time (like 2:30 AM on a March Sunday) "never existed" (because the clock sprang forward at 2:00 AM) or is "ambiguous" (because the clock fell back at 2:00 AM, so 1:30 AM happened twice).

The ORA_DST_AFFECTED function checks for this and returns:

• 1 if the timestamp is affected by the change (it will cause an error).
• 0 if the timestamp is not affected and is safe.

ORA_DST_AFFECTED Function Syntax

The syntax for ORA_DST_AFFECTED is:

ORA_DST_AFFECTED(timestamp_with_time_zone_expr)

Let's break that down:

• timestamp_with_time_zone_expr: The TIMESTAMP WITH TIME ZONE value or column you want to check.

Important: This is a DBA-Only Function

You cannot run this function in a normal SQL session. It is designed to be run only during a specific database maintenance window.

If you try to run it, you will get an ORA-30190 error: ORA-30190: ORA_DST_AFFECTED can be issued only during a TSTZ upgrade

A DBA must first start the upgrade process (e.g., by running DBMS_DST.BEGIN_PREPARE or DBMS_DST.BEGIN_UPGRADE) before this function becomes active.

The examples below are conceptual and show how a DBA would use them during that maintenance process.

Oracle ORA_DST_AFFECTED Function Examples

Example 1: How a DBA Checks for All Affected Data

This is the most common use case. After starting the time zone upgrade process, a DBA needs to find all rows in a table that will be invalid under the new rules.

Let's say they have an events table with an event_time column.

Query (Run by DBA during DST upgrade):

-- Find all rows that need to be fixed
SELECT 
  event_id, 
  event_time
FROM 
  events
WHERE 
  ORA_DST_AFFECTED(event_time) = 1;

Result: (The query would return a list of all events whose timestamps are now invalid, allowing the DBA to fix them manually.)

Example 2: How a DBA Checks a Specific "Problem" Time

Imagine a DBA knows that a new DST rule affects the 'US/Eastern' time zone on March 12, 2023, making the 2:00 AM hour non-existent. They can test a specific value.

Query (Run by DBA during DST upgrade):

SELECT 
  ORA_DST_AFFECTED(
    TO_TIMESTAMP_TZ('2023-03-12 02:30:00 US/Eastern', 
                    'YYYY-MM-DD HH24:MI:SS TZR')
  ) AS "Is_Time_Affected"
FROM DUAL;

Result:

Is_Time_Affected
----------------
               1

The result 1 confirms that this timestamp is now invalid ("affected") under the new time zone file rules.