DBMS_OUTPUT: Re-ENABLE should preserve buffer content like Oracle

[rophy]

Current Behavior

When DBMS_OUTPUT.ENABLE() is called while output is already enabled, IvorySQL clears the existing buffer content.

DBMS_OUTPUT.ENABLE();
DBMS_OUTPUT.PUT_LINE('First');
DBMS_OUTPUT.ENABLE();  -- Clears buffer
DBMS_OUTPUT.GET_LINE(line, status);
-- status = 1 (no lines available)

Expected Behavior (Oracle)

Oracle preserves existing buffer content when ENABLE() is called multiple times:

DBMS_OUTPUT.ENABLE();
DBMS_OUTPUT.PUT_LINE('First');
DBMS_OUTPUT.ENABLE();  -- Preserves buffer
DBMS_OUTPUT.GET_LINE(line, status);
-- line = 'First', status = 0

Impact

Medium. Applications that call ENABLE() multiple times during execution may see different behavior when migrating from Oracle.

References

• Documented in commit 868a774: "Re-ENABLE clears buffer vs preserves"
• Oracle compatibility test 3.3 in contrib/ivorysql_ora/sql/ora_dbms_output.sql
• Oracle DBMS_OUTPUT.ENABLE documentation

Workaround

Always call ENABLE() only once at the start of execution to ensure consistent behavior between IvorySQL and Oracle.