Informix Error -877: Isolation Level previously set by "Set Transaction".

Reasons / Common Causes

-877 fires when SET ISOLATION is issued after a SET TRANSACTION statement already established the isolation level for the current transaction — per the official guidance, once SET TRANSACTION has set it, SET ISOLATION shouldn't be used to change it.

1. SET ISOLATION issued after SET TRANSACTION already set the isolation level in the same transaction, per the official guidance — the direct, only cause.
2. Code mixing the older SET ISOLATION statement with the newer, more comprehensive SET TRANSACTION statement, not realizing the two shouldn't both be used to set isolation level within the same transaction.

Solutions / Resolution

1. Don't use a SET ISOLATION statement after SET TRANSACTION, per the official guidance — this is a hard restriction, not a configurable option.
2. Set the isolation level once, via SET TRANSACTION, per the official guidance's implied approach, and don't attempt to change it again within the same transaction using SET ISOLATION.
3. If a different isolation level is genuinely needed, start a new transaction and set it via SET TRANSACTION from the beginning of that new transaction.

Examples

The disallowed attempt

BEGIN WORK;
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
SET ISOLATION TO REPEATABLE READ;
-- -877: isolation level was already set by SET TRANSACTION

Corrected — set it once via SET TRANSACTION

BEGIN WORK;
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;

Diagnostic Checks

1. Check whether SET TRANSACTION already set the isolation level earlier in the same transaction, before a SET ISOLATION statement is reached.

Related Errors / Related Topics

• -876 — "Cannot issue SET TRANSACTION once a transaction has started." A related SET TRANSACTION timing restriction, about the statement itself coming too late rather than a conflicting SET ISOLATION.
• -875 — "Incompatible Access Mode and Isolation Level." A related SET TRANSACTION restriction, on which isolation levels can combine with which access modes.

Set the isolation level once via SET TRANSACTION — don't follow it with SET ISOLATION in the same transaction.