My Oracle Support Banner

TABLE ACCESS FULL When Modifying A Column From CHAR To VARCHAR2 And Takes More Time (Doc ID 2748995.1)

Last updated on JULY 25, 2023

Applies to:

Oracle Database - Enterprise Edition - Version and later
Information in this document applies to any platform.


Starting from Oracle 18c, there is a serious performance degradation when doing "alter table modify column".

Test case:

Up to Oracle RDBMS 12.2 this statement was executing instantly (when the column is nullable).

Starting from Oracle 18c it will execute in many hours on large table, which is not acceptable.

Comparing the SQL Trace (10046) of 18c/19c and 12.2 we can see that on newer versions it started doing a FULL TABLE SCAN:


 Upgrade from 12.2 to 18c


To view full details, sign in with your My Oracle Support account.

Don't have a My Oracle Support account? Click to get started!

In this Document

My Oracle Support provides customers with access to over a million knowledge articles and a vibrant support community of peers and Oracle experts.