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Description

This error indicates that a `CREATE TABLE ... IGNORE SELECT` statement is considered unsafe for replication. It occurs because the `IGNORE` clause depends on the implicit order in which rows are retrieved by the `SELECT` query, and this order cannot be guaranteed to be consistent across source and replica servers, potentially leading to data inconsistencies.

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Error Message

CREATE... IGNORE SELECT is unsafe because the order in which rows are retrieved by the SELECT determines which (if any) rows are ignored. This order cannot be predicted and may differ on source and the replica.

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Known Causes

3 known causes

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Unpredictable Row Order

The `SELECT` statement's implicit row retrieval order, which dictates which rows are ignored, is not guaranteed to be consistent between the source and replica servers.

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Statement-Based Replication (SBR)

When using Statement-Based Replication, MySQL cannot safely replicate `CREATE... IGNORE SELECT` as it cannot ensure the exact same outcome on the replica due to order dependency.

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Non-Deterministic `IGNORE` Logic

The `IGNORE` clause, which discards rows based on the first encountered duplicate, introduces non-determinism when the underlying `SELECT` order is not strictly defined and replicable.

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Solutions

4 solutions available

1. Explicitly Define Row Order with ORDER BY easy

Ensure predictable row processing by adding an ORDER BY clause to your SELECT statement.

1

Identify the `CREATE IGNORE SELECT` statement that is causing the error.

2

Modify the `SELECT` part of the statement to include an `ORDER BY` clause. This clause should specify columns that guarantee a unique and consistent order for the rows being selected. If there's a primary key or a combination of columns that uniquely identifies each row, use those.

CREATE IGNORE TABLE target_table SELECT * FROM source_table ORDER BY primary_key_column, another_unique_column;

3

Execute the modified `CREATE IGNORE SELECT` statement.

2. Remove IGNORE Clause and Handle Duplicates Explicitly medium

Disable the `IGNORE` keyword and implement logic to manage duplicate rows after insertion.

1

Locate the `CREATE IGNORE SELECT` statement.

2

Remove the `IGNORE` keyword from the `CREATE` statement. This will cause the statement to fail if duplicate keys are encountered, allowing you to address them.

CREATE TABLE target_table SELECT * FROM source_table;

3

If the statement fails due to duplicate entries, you will need to implement a strategy to handle them. This might involve:
- Using `INSERT IGNORE` on a subsequent `INSERT` statement if you are inserting into an existing table.
- Performing a `DELETE` operation to remove duplicates from the target table based on your defined uniqueness constraints.
- Using `INSERT ... ON DUPLICATE KEY UPDATE` if you want to update existing rows instead of ignoring them.

/* Example for handling duplicates after initial insert failure */
-- Find and delete duplicates based on a unique constraint
DELETE t1 FROM target_table t1
INNER JOIN target_table t2
WHERE t1.id < t2.id AND t1.unique_column = t2.unique_column;

3. Use Temporary Table for Intermediate Processing medium

Process and sort data in a temporary table before inserting into the final destination.

1

Create a temporary table that mirrors the structure of your target table.

CREATE TEMPORARY TABLE temp_target_table LIKE target_table;

2

Insert data from the source table into the temporary table, ensuring a defined order.

INSERT INTO temp_target_table SELECT * FROM source_table ORDER BY primary_key_column, another_unique_column;

3

Now, insert data from the temporary table into your actual target table. You can use `INSERT IGNORE` here if you've already handled potential duplicates in the temporary table or if you still want to ignore them at this final stage.

INSERT IGNORE INTO target_table SELECT * FROM temp_target_table;

4

Drop the temporary table when it's no longer needed.

DROP TEMPORARY TABLE temp_target_table;

4. Review and Refactor Data Transfer Logic advanced

Re-evaluate the necessity of `CREATE IGNORE SELECT` and explore alternative data synchronization methods.

1

Understand the exact purpose of the `CREATE IGNORE SELECT` statement. Is it intended for initial table creation with data, or for ongoing data synchronization?

2

If this is for initial table creation and you're encountering duplicates that you want to ignore, consider creating the table first without any data, and then using `INSERT IGNORE` to populate it. This separates the schema creation from the data insertion.

CREATE TABLE target_table (
    -- column definitions
);
INSERT IGNORE INTO target_table SELECT * FROM source_table;

3

For ongoing data synchronization, explore dedicated replication solutions (e.g., MySQL Replication, Galera Cluster) or ETL tools. These tools are designed to handle data consistency and conflict resolution more robustly.

4

If the `IGNORE` clause is used to handle expected duplicate entries due to the nature of the data, ensure that the underlying uniqueness constraints (e.g., primary keys, unique indexes) are correctly defined on the target table to leverage these constraints effectively.

ALTER TABLE target_table ADD UNIQUE INDEX unique_idx (column1, column2);

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Related Errors

5 related errors

MySQL Error 1717: Unsafe CREATE IGNORE SELECT