Oracle DBMS_OUTPUT: Multiple ENABLE() Calls Behavior

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Oracle DBMS_OUTPUT: Multiple ENABLE() Calls Behavior

Tested on: Oracle Database 23.26 Free
Date: 2025-12-25

Overview

This report documents the behavior when DBMS_OUTPUT.ENABLE() is called multiple times within a session, based on empirical testing against a real Oracle database.

Oracle Documentation states: "If there are multiple calls to ENABLE, then buffer_size is the last of the values specified."

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1. Basic Behavior

Behavior	Result
Multiple ENABLE calls	Buffer content is preserved (not cleared)
Buffer size	Last specified value becomes the limit
Only DISABLE clears	DISABLE() is the only way to clear buffer content

DBMS_OUTPUT.DISABLE;
DBMS_OUTPUT.ENABLE(5000);
DBMS_OUTPUT.PUT_LINE('Line 1');
DBMS_OUTPUT.ENABLE(10000);  -- Buffer NOT cleared
DBMS_OUTPUT.PUT_LINE('Line 2');
-- Output: Both lines appear

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2. Shrinking Buffer Size

When calling ENABLE(smaller_size) after ENABLE(larger_size):

2.1 Empty Buffer

New smaller limit is enforced strictly.

DBMS_OUTPUT.DISABLE;
DBMS_OUTPUT.ENABLE(5000);
DBMS_OUTPUT.ENABLE(2500);  -- Shrink while empty
DBMS_OUTPUT.PUT_LINE(RPAD('X', 2600, 'X'));  -- FAILS: exceeds 2500

2.2 Buffer Content < New Limit

New limit is enforced for total (existing + new).

DBMS_OUTPUT.DISABLE;
DBMS_OUTPUT.ENABLE(5000);
DBMS_OUTPUT.PUT_LINE(RPAD('X', 1000, 'X'));  -- 1000 bytes
DBMS_OUTPUT.ENABLE(2000);  -- Shrink to 2000
DBMS_OUTPUT.PUT_LINE(RPAD('Y', 1500, 'Y'));  -- FAILS: 1000+1500 > 2000

2.3 Buffer Content >= New Limit (Key Finding)

Additive capacity behavior: When existing content already exceeds the new limit, Oracle expands capacity to approximately first_size + second_size.

DBMS_OUTPUT.DISABLE;
DBMS_OUTPUT.ENABLE(4000);
DBMS_OUTPUT.PUT_LINE(RPAD('X', 3500, 'X'));  -- 3500 > 3000
DBMS_OUTPUT.ENABLE(3000);  -- Shrink, but content exceeds new limit
-- Effective capacity becomes ~7000 (4000 + 3000)
DBMS_OUTPUT.PUT_LINE(RPAD('Y', 3000, 'Y'));  -- OK: total ~6500 < 7000

First ENABLE	Second ENABLE	Content before shrink	Effective Capacity
3000	2500	> 2500	~5500
4000	3000	> 3000	~7000

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3. Effect of GET_LINES() After Shrinking

After GET_LINES() flushes the buffer, the additive capacity is lost. The limit reverts to the last ENABLE value.

DBMS_OUTPUT.DISABLE;
DBMS_OUTPUT.ENABLE(4000);
DBMS_OUTPUT.PUT_LINE(RPAD('X', 3500, 'X'));
DBMS_OUTPUT.ENABLE(3000);  -- Additive: capacity ~7000
DBMS_OUTPUT.GET_LINES(lines, numlines);  -- Flush
-- Capacity now reverts to 3000
DBMS_OUTPUT.PUT_LINE(RPAD('Y', 2999, 'Y'));  -- OK
DBMS_OUTPUT.PUT_LINE('Z');  -- OK (total 3000)
DBMS_OUTPUT.PUT_LINE('A');  -- FAILS: exceeds 3000

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4. Buffer Reuse After GET_LINES()

GET_LINES() frees buffer space. The limit applies to current contents, not cumulative lifetime writes.

DBMS_OUTPUT.DISABLE;
DBMS_OUTPUT.ENABLE(2000);
DBMS_OUTPUT.PUT_LINE(RPAD('X', 1500, 'X'));  -- 1500
DBMS_OUTPUT.GET_LINES(lines, numlines);      -- Flush
DBMS_OUTPUT.PUT_LINE(RPAD('Y', 1500, 'Y'));  -- OK: buffer was cleared
DBMS_OUTPUT.GET_LINES(lines, numlines);      -- Flush again
DBMS_OUTPUT.PUT_LINE(RPAD('Z', 1500, 'Z'));  -- OK: can reuse indefinitely

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5. Summary Table

Scenario	Buffer Cleared?	Effective Limit
ENABLE(A) then ENABLE(B) (empty buffer)	No	B
ENABLE(A) -> write -> ENABLE(B) where B > content	No	B
ENABLE(A) -> write -> ENABLE(B) where B <= content	No	~A+B (additive)
After GET_LINES() flush	Yes (content)	Last ENABLE value
DISABLE()	Yes	N/A (disabled)

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6. Practical Implications

1. Don't rely on shrinking: Calling ENABLE(smaller) doesn't reclaim memory or enforce the smaller limit if content already exceeds it.

2. Use DISABLE to reset: If you need a clean slate, call DISABLE() first.

3. GET_LINES resets capacity: After flushing, the additive capacity bonus is lost.

4. Application code should avoid ENABLE/DISABLE: Per Oracle documentation, these calls can interfere with tools like SQL*Plus that manage output display.

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7. Buffer Size Constraints

Parameter	Value
Minimum size	2,000 bytes
Maximum size	1,000,000 bytes
Default	20,000 bytes
Unlimited	NULL

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References

• Oracle DBMS_OUTPUT.ENABLE Documentation