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Description

This error indicates that an `INSERT... SELECT... ON DUPLICATE KEY UPDATE` statement cannot be safely replicated in a statement-based replication environment. The non-deterministic order of rows returned by the `SELECT` clause can lead to different outcomes on the master and slave, causing data inconsistency. It typically occurs when using statement-based logging for replication.

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Error Message

INSERT... SELECT... ON DUPLICATE KEY UPDATE is unsafe because the order in which rows are retrieved by the SELECT determines which (if any) rows are updated. This order cannot be predicted and may differ on master and the slave.

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Known Causes

3 known causes

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Unpredictable SELECT Order

The `SELECT` part of the `INSERT... SELECT... ON DUPLICATE KEY UPDATE` statement lacks an `ORDER BY` clause, causing the order of retrieved rows to be non-deterministic. This unpredictability makes the `ON DUPLICATE KEY UPDATE` logic unsafe for replication.

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Statement-Based Replication in Use

The MariaDB server is configured to use statement-based binary logging (`binlog_format='STATEMENT'`). This logging format records the exact SQL statements, which can lead to replication inconsistencies with non-deterministic queries.

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Missing ORDER BY in SELECT

The `SELECT` subquery within the `INSERT... SELECT... ON DUPLICATE KEY UPDATE` statement does not include an `ORDER BY` clause. Without a defined order, the database cannot guarantee consistent row processing, which is critical for replication safety.

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Solutions

3 solutions available

1. Ensure Deterministic SELECT Order with ORDER BY easy

Add an ORDER BY clause to the SELECT statement to guarantee a consistent row retrieval order.

1

Analyze the `INSERT ... SELECT ... ON DUPLICATE KEY UPDATE` statement that is causing the error. Identify the `SELECT` part.

2

Modify the `SELECT` statement to include an `ORDER BY` clause. This clause should specify one or more columns that uniquely identify rows or guarantee a consistent ordering. Ideally, use a primary key or a unique index.

INSERT INTO target_table (col1, col2, col3)
SELECT colA, colB, colC
FROM source_table
WHERE condition
ORDER BY unique_identifier_column ASC; -- Or DESC, depending on desired order
ON DUPLICATE KEY UPDATE col2 = VALUES(col2), col3 = VALUES(col3);

3

Test the modified statement on your development or staging environment to ensure it executes without error and produces the expected results.

2. Separate INSERT and UPDATE Operations medium

Split the operation into two distinct statements: an INSERT and then a conditional UPDATE.

1

Identify the `INSERT ... SELECT ... ON DUPLICATE KEY UPDATE` statement.

2

Create a temporary table or use a Common Table Expression (CTE) to store the results of the `SELECT` statement. This ensures the data is materialized before any updates.

WITH selected_data AS (
  SELECT colA, colB, colC
  FROM source_table
  WHERE condition
  -- No ORDER BY needed here if we are just materializing
)
INSERT INTO target_table (col1, col2, col3)
SELECT colA, colB, colC
FROM selected_data
ON DUPLICATE KEY UPDATE col2 = VALUES(col2), col3 = VALUES(col3);

3

If a CTE is not an option or preferred, use a temporary table. First, insert the data into the temporary table, then perform the `ON DUPLICATE KEY UPDATE` from the temporary table.

CREATE TEMPORARY TABLE temp_source_data AS
SELECT colA, colB, colC
FROM source_table
WHERE condition;

INSERT INTO target_table (col1, col2, col3)
SELECT colA, colB, colC
FROM temp_source_data
ON DUPLICATE KEY UPDATE col2 = VALUES(col2), col3 = VALUES(col3);

DROP TEMPORARY TABLE temp_source_data;

4

Alternatively, perform a direct `INSERT IGNORE` or `INSERT ... SELECT` and then a separate `UPDATE` statement for rows that already exist. This requires more complex logic to identify which rows need updating.

INSERT INTO target_table (col1, col2, col3)
SELECT colA, colB, colC
FROM source_table
WHERE condition
ON DUPLICATE KEY UPDATE col1 = col1; -- Dummy update to prevent duplicates if needed, or use INSERT IGNORE

-- Then, a separate update statement if needed, carefully matching rows.

3. Re-evaluate Replication Strategy for Unsafe Statements advanced

Modify replication settings to allow or handle unsafe statements, or reconsider the statement's design.

1

Understand the implications of `log_bin_trust_function_creators` and `slave_exec_mode`. Setting `log_bin_trust_function_creators = 1` on the master allows stored functions/triggers that might be involved in unsafe operations to be logged. `slave_exec_mode = 'IDEMPOTENT'` on the slave can help, but it's a workaround and not a fundamental fix for the ordering issue.

2

Consider disabling `slave_exec_mode` and instead focusing on making the statement deterministic on the master. This is the preferred approach.

3

If the application logic absolutely requires this type of statement and cannot be refactored, and you are willing to accept potential inconsistencies or performance impacts, you could consider disabling `slave_exec_mode` or setting `slave_exec_mode = 'IDEMPOTENT'` on the slave. **This is generally NOT recommended for production environments as it can mask underlying data inconsistencies.**

SET GLOBAL slave_exec_mode = 'IDEMPOTENT'; -- On the slave server

4

The most robust solution is to refactor the application logic to avoid such unsafe replication statements. This often involves using the deterministic approaches outlined in other solutions.

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Related Errors

5 related errors

MariaDB Error 1715: Unsafe Replication Update Order