RETURNING Clause in Prepared Statements Fails to Return Values After ON CONFLICT

Issue Overview: Prepared Statements with RETURNING Clause Not Returning Rows After ON CONFLICT DO UPDATE

The core issue revolves around the behavior of SQLite’s RETURNING clause when used within prepared statements that involve ON CONFLICT DO UPDATE logic. The user observes that when executing an INSERT ... ON CONFLICT DO UPDATE ... RETURNING query via a prepared statement, the RETURNING clause does not produce a result row, whereas the same query executed in "unprepared" mode (e.g., via sqlite3_exec() or the SQLite shell) works as expected. Specifically:

• When the query is executed as a single unprepared statement (e.g., via sqlite3_exec()), the RETURNING clause returns the my_pk value of the inserted or updated row.
• When the same query is executed via a prepared statement (using sqlite3_prepare_v2(), sqlite3_bind_*(), and sqlite3_step()), the sqlite3_step() function returns SQLITE_DONE instead of SQLITE_ROW, and no data is retrievable via sqlite3_column_*() functions.

The table schema includes a unique constraint (UNIQUE(my_uxi)), and the ON CONFLICT clause is designed to update the conflicting row. The RETURNING clause is intended to return the primary key (my_pk) of the affected row, whether it was inserted or updated. The discrepancy between prepared and unprepared execution suggests a misunderstanding of how SQLite processes multi-statement transactions, prepares individual statements, and handles result sets from data-modifying queries.

Possible Causes: Misuse of Prepared Statements, Syntax Errors, and Transaction Boundaries

1. Multi-Statement Transactions in a Single Prepared Statement

SQLite’s sqlite3_prepare_v2() function compiles only the first SQL statement in the input string. If the input contains multiple statements (e.g., BEGIN TRANSACTION; INSERT ...; END TRANSACTION;), sqlite3_prepare_v2() will prepare only the BEGIN TRANSACTION statement, ignoring subsequent statements. Executing this prepared statement will start a transaction but will not execute the INSERT or RETURNING logic. This explains why sqlite3_step() returns SQLITE_DONE immediately: it is reporting the completion of the BEGIN command, not the INSERT.

2. Incorrect Binding or Typographical Errors in the Prepared Statement

The original discussion revealed a mismatch between the column name in the ON CONFLICT clause (my_key) and the actual unique column (my_uxi). While this was corrected in later replies, such errors can cause prepared statements to fail during compilation (returning SQLITE_ERROR). If the user’s code does not check for errors during preparation, the subsequent sqlite3_step() call might execute a malformed or unintended statement, leading to unexpected behavior.

3. Misunderstanding Prepared Statement Execution Workflow

Prepared statements in SQLite require explicit handling of intermediate result states. For INSERT ... RETURNING queries, sqlite3_step() returns SQLITE_ROW for each row produced by RETURNING, followed by SQLITE_DONE after the last row. If the prepared statement includes only the INSERT (without surrounding BEGIN/END), but the user fails to call sqlite3_step() repeatedly or handle SQLITE_ROW correctly, the RETURNING data might be missed.

4. Transaction Isolation and Auto-Commit Mode

SQLite operates in auto-commit mode by default. Explicit transactions (BEGIN/COMMIT) disable auto-commit. If the unprepared execution implicitly uses auto-commit (e.g., via sqlite3_exec()), while the prepared statement explicitly wraps the INSERT in a transaction, differences in locking or isolation could theoretically affect visibility of the RETURNING results. However, this is unlikely to be the root cause here.

5. SQLite Version-Specific Behavior

The user reported using SQLite 3.37.0. While RETURNING was introduced in SQLite 3.35.0 (2021-03-12), earlier versions would reject such syntax outright. Version-specific bugs in prepared statement execution are possible but unlikely in this case, as the behavior aligns with documented SQLite functionality.

Troubleshooting Steps, Solutions & Fixes: Ensuring Correct Statement Preparation and Execution

Step 1: Isolate the INSERT Statement from Transaction Commands

Problem: Mixing BEGIN TRANSACTION, INSERT, and END TRANSACTION in a single input to sqlite3_prepare_v2() causes only the BEGIN to be prepared.
Solution: Prepare and execute each SQL statement separately:

1. Prepare and execute BEGIN TRANSACTION;
2. Prepare and execute the INSERT ... RETURNING; statement.
3. Prepare and execute COMMIT; or END TRANSACTION;.

Example Code:

sqlite3 *db;
sqlite3_stmt *stmt;
int rc;

// Begin transaction
rc = sqlite3_prepare_v2(db, "BEGIN TRANSACTION;", -1, &stmt, NULL);
if (rc != SQLITE_OK) { /* handle error */ }
rc = sqlite3_step(stmt);
if (rc != SQLITE_DONE) { /* handle error */ }
sqlite3_finalize(stmt);

// Prepare INSERT ... RETURNING
rc = sqlite3_prepare_v2(db, "INSERT INTO MyTable (my_pk, my_value) VALUES (?, ?) ON CONFLICT(my_uxi) DO UPDATE SET my_value = ? RETURNING my_pk;", -1, &stmt, NULL);
if (rc != SQLITE_OK) { /* handle error */ }

// Bind parameters
sqlite3_bind_int(stmt, 1, 111);
sqlite3_bind_text(stmt, 2, "A value", -1, SQLITE_STATIC);
sqlite3_bind_text(stmt, 3, "A NewValue", -1, SQLITE_STATIC);

// Execute INSERT and handle RETURNING
rc = sqlite3_step(stmt);
if (rc == SQLITE_ROW) {
    int64_t my_pk = sqlite3_column_int64(stmt, 0);
    // Process my_pk...
} else if (rc == SQLITE_DONE) {
    // No row returned (unexpected in this case)
} else {
    // Handle error
}
sqlite3_finalize(stmt);

// Commit transaction
rc = sqlite3_prepare_v2(db, "COMMIT;", -1, &stmt, NULL);
if (rc != SQLITE_OK) { /* handle error */ }
rc = sqlite3_step(stmt);
if (rc != SQLITE_DONE) { /* handle error */ }
sqlite3_finalize(stmt);

Step 2: Verify Column Names and SQL Syntax

Ensure that all column names in the ON CONFLICT clause and elsewhere match the schema. Use the SQLite shell to test the query interactively:

sqlite> CREATE TABLE MyTable(
   ...>   my_pk INTEGER PRIMARY KEY AUTOINCREMENT,
   ...>   my_uxi NUMERIC NULL DEFAULT 0,
   ...>   my_value CHAR(256),
   ...>   UNIQUE(my_uxi)
   ...> );
sqlite> INSERT INTO MyTable (my_pk, my_value) VALUES (111, 'A value')
   ...> ON CONFLICT(my_uxi) DO UPDATE SET my_value = 'A NewValue'
   ...> RETURNING my_pk;

Step 3: Check Prepared Statement Compilation Errors

Always check the return value of sqlite3_prepare_v2() and sqlite3_step(), and log any error messages:

rc = sqlite3_prepare_v2(db, sql_query, -1, &stmt, NULL);
if (rc != SQLITE_OK) {
    fprintf(stderr, "Preparation error: %s\n", sqlite3_errmsg(db));
    return;
}

Step 4: Handle Multiple Rows from RETURNING

If the RETURNING clause could produce multiple rows (unlikely in this case), loop over SQLITE_ROW results:

while ((rc = sqlite3_step(stmt)) == SQLITE_ROW) {
    int64_t my_pk = sqlite3_column_int64(stmt, 0);
    // Process each row...
}
if (rc != SQLITE_DONE) { /* handle error */ }

Step 5: Use sqlite3_exec() for Unprepared Execution Comparison

To replicate the "unprepared" workflow, use sqlite3_exec() with the same SQL. This helps isolate whether the issue is specific to prepared statements or a broader SQL logic error:

char *errmsg = NULL;
rc = sqlite3_exec(db,
    "INSERT INTO MyTable (my_pk, my_value) VALUES (111, 'A value') "
    "ON CONFLICT(my_uxi) DO UPDATE SET my_value = 'A NewValue' "
    "RETURNING my_pk;",
    callback,  // Define a callback function to process results
    NULL,
    &errmsg
);
if (rc != SQLITE_OK) {
    fprintf(stderr, "Execution error: %s\n", errmsg);
    sqlite3_free(errmsg);
}

Step 6: Ensure Autocommit Mode Compatibility

If transactions are unnecessary, remove BEGIN/COMMIT and rely on SQLite’s auto-commit mode. This simplifies the code and avoids transaction-related pitfalls:

// Directly prepare and execute the INSERT ... RETURNING statement
rc = sqlite3_prepare_v2(db, "INSERT ... RETURNING my_pk;", -1, &stmt, NULL);
// Bind, step, and process as before...

Step 7: Update SQLite and Validate Environment

Ensure the SQLite library version is 3.35.0 or newer (required for RETURNING). Check for environment-specific issues, such as custom SQLite builds with disabled features or conflicting extensions.

Final Solution Summary

The primary cause of the RETURNING clause not producing results in prepared statements is the inclusion of BEGIN TRANSACTION and END TRANSACTION within the same input string passed to sqlite3_prepare_v2(), causing only the BEGIN to be executed. By preparing and executing each statement separately, ensuring correct syntax, and properly handling result rows, the RETURNING clause will function as expected in both prepared and unprepared execution modes.