In-Memory SQLite Database Not Saved via VACUUM INTO: Causes and Solutions

Understanding VACUUM INTO Failures with In-Memory Databases

Issue Overview: Silent Failure When Saving In-Memory Databases via VACUUM INTO

The problem arises when attempting to save an in-memory SQLite database to a file using the VACUUM INTO command. While the same operation succeeds when applied to an on-disk database, it fails silently for in-memory databases: no error is reported, but no file is created. The user’s code checks for errors incorrectly, interpreting the SQLITE_DONE status (a normal completion signal) as an error. However, even after correcting error handling, the VACUUM INTO command does not generate a file for in-memory databases. This behavior stems from fundamental limitations in how SQLite handles in-memory databases and the VACUUM command’s requirements.

Key observations:

• VACUUM INTO works for on-disk databases but fails for in-memory databases.
• The error message no more rows available (corresponding to SQLITE_DONE) is misinterpreted as a failure.
• Resource leaks (unfreed statements, unmanaged strings) compound the issue.

Root Causes: Why VACUUM INTO and In-Memory Databases Conflict

Three primary factors contribute to this issue:

1. Misuse of VACUUM INTO with In-Memory Databases
   The VACUUM INTO command is designed to clone or rebuild the main database into a new file. However, when the main database is in-memory, SQLite’s internal implementation of VACUUM cannot interact with it in the same way as a disk-based database. Specifically:

   • In-memory databases lack a backing file, making it impossible for VACUUM INTO to resolve the target path relative to the main database’s storage.
   • Temporary or in-memory databases are excluded from certain durability guarantees and file operations, which VACUUM implicitly relies on.

2. Incorrect Error Handling in Code
   The sqlite3_step() function returns SQLITE_DONE when a command finishes successfully. The user’s code incorrectly treats any return value other than SQLITE_OK as an error, leading to false positives. This misinterprets the normal completion status as a failure, creating confusion about whether the operation succeeded.

3. Resource Management Oversights
   The original code fails to:

   • Properly finalize SQL statements in all code paths (leaking memory).
   • Free the string returned by sqlite3_expanded_sql(), which must be manually released with sqlite3_free().

Resolving the Issue: Step-by-Step Fixes and Alternatives

To address the problem, follow these steps:

Step 1: Correct Error Handling for sqlite3_step()

Modify the code to recognize SQLITE_DONE as a success status for VACUUM operations:

int save_database(sqlite3 *db, const char *fn) {
    sqlite3_stmt *stmt = NULL;
    char *expanded_sql = NULL;
    int rc = 0;

    // Prepare the VACUUM statement
    if (sqlite3_prepare_v2(db, "VACUUM INTO ?;", -1, &stmt, NULL) != SQLITE_OK) {
        fprintf(stderr, "Prepare error: %s\n", sqlite3_errmsg(db));
        rc = 1;
        goto cleanup;
    }

    // Bind the filename parameter
    if (sqlite3_bind_text(stmt, 1, fn, -1, SQLITE_STATIC) != SQLITE_OK) {
        fprintf(stderr, "Bind error: %s\n", sqlite3_errmsg(db));
        rc = 1;
        goto cleanup;
    }

    // Debugging: Print the expanded SQL (remember to free it!)
    expanded_sql = sqlite3_expanded_sql(stmt);
    printf("%s\n", expanded_sql);

    // Execute the statement
    int step_rc = sqlite3_step(stmt);
    if (step_rc != SQLITE_DONE) {
        fprintf(stderr, "Step error (%d): %s\n", step_rc, sqlite3_errmsg(db));
        rc = 1;
    }

cleanup:
    // Free resources
    sqlite3_free(expanded_sql);
    sqlite3_finalize(stmt);
    return rc;
}

This revised code:

• Uses SQLITE_DONE as the success indicator.
• Properly frees the expanded SQL string.
• Ensures the statement is finalized even if errors occur.

Step 2: Avoid VACUUM INTO for In-Memory Databases

Since VACUUM INTO does not support in-memory databases, use one of these alternatives:

Option A: Use the Backup API
SQLite’s Backup API is designed to clone databases efficiently:

#include <sqlite3.h>

int save_database(sqlite3 *src_db, const char *fn) {
    sqlite3 *dst_db = NULL;
    sqlite3_backup *backup = NULL;
    int rc = 0;

    // Open the target database
    if (sqlite3_open(fn, &dst_db) != SQLITE_OK) {
        fprintf(stderr, "Failed to open destination: %s\n", sqlite3_errmsg(dst_db));
        rc = 1;
        goto cleanup;
    }

    // Initialize the backup
    backup = sqlite3_backup_init(dst_db, "main", src_db, "main");
    if (!backup) {
        fprintf(stderr, "Backup init failed: %s\n", sqlite3_errmsg(dst_db));
        rc = 1;
        goto cleanup;
    }

    // Perform the backup
    do {
        rc = sqlite3_backup_step(backup, -1); // Copy all pages
    } while (rc == SQLITE_OK || rc == SQLITE_BUSY || rc == SQLITE_LOCKED);

    if (rc != SQLITE_DONE) {
        fprintf(stderr, "Backup error: %s\n", sqlite3_errmsg(dst_db));
        rc = 1;
        goto cleanup;
    }

    // Finalize the backup
    sqlite3_backup_finish(backup);
    backup = NULL;

cleanup:
    if (backup) sqlite3_backup_finish(backup);
    if (dst_db) sqlite3_close(dst_db);
    return rc;
}

Advantages:

• Works for both in-memory and on-disk databases.
• Efficiently copies all data, including indexes and schema.

Option B: Attach a Disk Database and Copy Data
Attach the target file as a secondary database and manually copy tables:

ATTACH 'graphcopy.db' AS diskdb;
CREATE TABLE diskdb.nodes AS SELECT * FROM main.nodes;
CREATE TABLE diskdb.edges AS SELECT * FROM main.edges;
-- Repeat for all tables, indexes, triggers, etc.
DETACH diskdb;

Drawbacks:

• Requires manual handling of all database objects.
• Error-prone for complex schemas.

Option C: Dump and Restore via .dump Command
Use sqlite3_exec() to generate a SQL dump and replay it against the new database:

int save_database(sqlite3 *src_db, const char *fn) {
    sqlite3 *dst_db = NULL;
    char *errmsg = NULL;
    int rc = 0;

    // Open the destination database
    if (sqlite3_open(fn, &dst_db) != SQLITE_OK) {
        fprintf(stderr, "Failed to open destination: %s\n", sqlite3_errmsg(dst_db));
        rc = 1;
        goto cleanup;
    }

    // Execute .dump on the source database
    rc = sqlite3_exec(src_db, "SELECT sql FROM sqlite_schema;", dump_callback, dst_db, &errmsg);
    if (rc != SQLITE_OK) {
        fprintf(stderr, "Dump failed: %s\n", errmsg);
        sqlite3_free(errmsg);
        rc = 1;
        goto cleanup;
    }

cleanup:
    if (dst_db) sqlite3_close(dst_db);
    return rc;
}

// Callback function to execute each SQL statement from the dump
int dump_callback(void *dst_db, int argc, char **argv, char **colname) {
    if (argc > 0 && argv[0]) {
        char *sql = argv[0];
        if (sqlite3_exec((sqlite3*)dst_db, sql, NULL, NULL, NULL) != SQLITE_OK) {
            return SQLITE_ABORT;
        }
    }
    return SQLITE_OK;
}

Drawbacks:

• Slower for large databases due to parsing and re-executing SQL.
• May fail for BLOB data or complex schemas.

Step 3: Validate and Test the Solution

After implementing one of the above fixes, verify the solution:

1. Test Case for In-Memory Database
   Open an in-memory database, create tables, insert data, and call save_database(). Confirm that the target file is created and contains all data.

2. Test Error Handling
   Pass invalid filenames (e.g., read-only paths) to ensure errors are reported correctly.

3. Profile Performance
   Compare the performance of the Backup API approach with the original VACUUM INTO method for on-disk databases to ensure no regressions.

Final Recommendation:
Use the Backup API for saving in-memory databases. It is robust, efficient, and avoids the pitfalls of VACUUM INTO. Correct error handling and resource management are critical to prevent leaks and false error reports.