How Does SQLite's UPSERT Handle Partial Updates and Inserts Efficiently?

Understanding SQLite's UPSERT Functionality

The UPSERT operation, a combination of INSERT and UPDATE, efficiently handles data modification by updating existing records or inserting new ones as needed. While SQLite directly supports UPSERT, optimizing its use for partial updates requires careful consideration.

SQLite's UPSERT Syntax

Introduced in SQLite version 3.24.0, the ON CONFLICT clause enables precise UPSERT control. For example:

INSERT OR REPLACE INTO table (id, name)
VALUES (1, 'John Foo') ON CONFLICT(id) DO UPDATE SET name = 'John Foo';

This updates the name field if a record with the matching id exists; otherwise, it inserts a new row.

Addressing Partial Updates

Managing partial updates—modifying only specific columns while leaving others untouched—presents a challenge. Consider this scenario: update Blob1 and Blob2, but leave Blob3 unchanged if the record exists; if it doesn't, set Blob3 to NULL.

Efficient Partial Update Solution

The ON CONFLICT clause provides an elegant solution:

INSERT INTO table (id, Blob1, Blob2, Blob3)
VALUES (1, 'Data1', 'Data2', NULL)
ON CONFLICT(id) DO UPDATE SET
  Blob1 = 'Data1',
  Blob2 = 'Data2';

This approach updates only Blob1 and Blob2 when a conflict (matching id) occurs. Importantly, if the record is absent, the INSERT sets Blob3 to NULL as specified.

Alternative Methods (Less Efficient)

Alternatively, a SELECT and REPLACE combination can achieve the same result:

SELECT name INTO @name FROM Employee WHERE id = 1;
REPLACE INTO Employee (id, role) VALUES (1, 'code monkey', @name);

This preserves the original name value by reading it beforehand. However, this method generally introduces more overhead compared to the ON CONFLICT approach.

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