Why are Oracle table/column/index names limited to 30 characters?

I believe it's the ANSI standard.

EDIT:

Actually, I think it's the SQL-92 standard.

A later version of the standard appears to optionally allow for 128 character names, but Oracle doesn't yet support this (or has partial support for it, insofar as it allows 30 characters. Hmmm.)

Search for "F391, Long identifiers" on this page... http://stanford.edu/dept/itss/docs/oracle/10g/server.101/b10759/ap_standard_sql001.htm

(Looking for a ref)

In addition to cagcowboy's point that it derives from the SQL standard (historically, I suspect that Oracle's decision lead to the SQL standard since Oracle predated the standardization of SQL), I would wager that a large part of the reluctance to allow longer identifiers comes from the realization that there are millions of DBAs with millions of custom scripts that all assume that identifiers are 30 characters long. Allowing every line of code that goes something like

  l_table_name VARCHAR2(30);BEGIN  SELECT table_name    INTO l_table_name    FROM dba_tables   WHERE ...

to suddenly break because the DBA 15 years ago used VARCHAR2(30) rather than DBA_TABLES.TABLE_NAME%TYPE in the script would cause massive revolt. I would wager that Oracle alone has thousands of places where this sort of thing has been done over the years in various packages and components. Retrofitting all that existing code to support longer identifiers would be a tremendous project that would almost certainly generate way more costs in developer time, QA time, and newly introduced bugs than it would generate benefits.

I was looking this up and found this question via Google, but also found out that as of Oracle 12c Release 2 (12.2), this is no longer strictly the case.(https://oracle-base.com/articles/12c/long-identifiers-12cr2)

At some point every DBA or developer will have hit a point where the 30 character limit for object names has caused a problem. This limit can be extremely painful when doing migration projects from SQL Server or MySQL to Oracle. In Oracle Database 12cR2, the maximum length of most identifiers is now 128 characters.

This is a new feature in 12.2, according to (http://blog.dbi-services.com/oracle-12cr2-long-identifiers/). According to that post, 12.1 was still limited to 30 characters.

Edit: Here's a link to the official Oracle documentation explaining the change. (https://docs.oracle.com/cloud/latest/exadataexpress-cloud/CSDBF/longer-identifier-names.htm#CSDBF-GUID-F4CA155F-5A37-4705-8443-0A8C9E3F875C)

Starting with Oracle Database 12c Release 2 (12.2), the maximum length of identifier names for most types of database objects has been increased to 128 bytes.