Oracle ORA_DST_CONVERT Function: A Simple Guide

The ORA_DST_CONVERT function in Oracle SQL is a highly specialized tool used by Database Administrators (DBAs) during a database time zone file upgrade.

This function is the "fix-it" tool that you use after ORA_DST_AFFECTED has found the problem data. Its job is to take an "old" timestamp and convert it to its correct representation under the new time zone rules. It also gives you options for handling conversion errors.

What is the ORA_DST_CONVERT Function in Oracle?

When a time zone file is updated, some existing TIMESTAMP WITH TIME ZONE data can become invalid (e.g., a time "never existed" or is "ambiguous").

The ORA_DST_CONVERT function attempts to convert these invalid timestamps. More importantly, it lets you decide what to do if an error occurs:

1. Suppress Errors (Default): If a timestamp can't be converted, the function can just return the original, problematic value.
2. Report Errors: You can tell the function to raise an error instead. This is useful for finding the exact rows that need to be manually fixed.

ORA_DST_CONVERT Function Syntax

The syntax for ORA_DST_CONVERT is:

ORA_DST_CONVERT(datetime_expr, [duplicate_time_error], [nonexisting_time_error])

Let's break that down:

• datetime_expr: The TIMESTAMP WITH TIME ZONE value or column you want to convert.
• [duplicate_time_error] (Optional):
  • 0 (Default): Suppresses "duplicate time" errors and returns the original value.
  • 1: Raises an error if a duplicate time is found.
• [nonexisting_time_error] (Optional):
  • 0 (Default): Suppresses "nonexisting time" errors and returns the original value.
  • 1: Raises an error if a nonexisting time is found.

Important: This is a DBA-Only Function

Like ORA_DST_AFFECTED, this function cannot be run in a normal SQL session. It is designed to be run only during a database upgrade, between the DBMS_DST.BEGIN_UPGRADE and DBMS_DST.END_UPGRADE procedures. Trying to run it outside this window will cause an error.

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

Oracle ORA_DST_CONVERT Function Examples

Example 1: Fixing Affected Data with ORA_DST_CONVERT

This is the most common use case. A DBA has already used ORA_DST_AFFECTED to find rows that have problems. Now, they will run an UPDATE statement to fix them using ORA_DST_CONVERT.

Query (Run by DBA during DST upgrade):

-- This query finds all affected rows and attempts to convert
-- them, suppressing any errors (the default behavior).
UPDATE 
  events
SET 
  event_time = ORA_DST_CONVERT(event_time)
WHERE 
  ORA_DST_AFFECTED(event_time) = 1;

Result: (This statement will update all convertible rows. Any rows that fail conversion (due to ambiguity) will remain unchanged, allowing the DBA to find and fix them manually afterward.)

Example 2: Finding Specific Errors with ORA_DST_CONVERT

Instead of suppressing errors, a DBA might first want to find the specific rows that will fail. They can do this by setting the error-handling arguments to 1 (report errors).

Query (Run by DBA during DST upgrade):

-- This SELECT statement will fail and report an error
-- as soon as it finds a value that cannot be converted.
SELECT 
  event_id,
  ORA_DST_CONVERT(event_time, 1, 1) AS "Converted_Time"
FROM 
  events
WHERE 
  ORA_DST_AFFECTED(event_time) = 1;

Result: (If a "nonexisting time" error is found, the query will stop and return an error message (like ORA-01878: specified field not found in datetime or interval), helping the DBA pinpoint the exact row causing the problem.)