Assertion Failure in SQLite JDBC Debug Build with UTF-16 Encoding and Function-Based WHERE Clause

Understanding the Assertion Failure in valueFromFunction During UTF-16 Query Execution

Issue Overview

The problem arises when executing a SQL query involving a BETWEEN clause with a function call (ABS('')) on a UTF-16 encoded database using a custom-compiled SQLite JDBC driver with -DSQLITE_DEBUG=1. The assertion failure occurs at sqlite3.c:81897 in the valueFromFunction method, triggered by a mismatch in text encoding (enc==pVal->enc) or unexpected memory flags. The error does not manifest in the SQLite CLI, even with identical schema and data. Key components include:

• A UTF-16 encoded database.
• An indexed TEXT column.
• Use of ANALYZE to generate statistics.
• A SELECT query comparing a text column to the result of ABS('') in a BETWEEN clause.

The assertion enforces that the encoding of a MEM structure (pVal->enc) matches the expected encoding (enc) or that the value is not a string. The failure suggests a violation of this rule under debug-mode checks in the JDBC driver.

Root Causes of Encoding Mismatch and Assertion Triggers

1. UTF-16 Encoding and Function Return Type Mismatch
   The ABS('') function returns a numeric value (integer or real), but the BETWEEN clause compares it to t0.c1, a TEXT column. SQLite performs implicit type conversions here, but the debug build strictly validates encoding consistency for string values. The empty string '' in BETWEEN is treated as a UTF-16 text value, while ABS('') resolves to 0 (a numeric type). When the comparison coerces these values, the internal representation may violate encoding expectations.

2. Index Usage and Query Planner Behavior
   The CREATE INDEX i0 ON t0(c1) and ANALYZE directive cause the query planner to use the index for the BETWEEN clause. Index comparisons require type and encoding consistency between the indexed column and the compared values. If the right-hand side of BETWEEN (ABS('')) is not properly coerced to the column’s encoding, the assertion fails.

3. JDBC Driver-Specific Handling of Encoded Values
   The Xerial JDBC driver replaces the default SQLite engine with a debug build. Debug builds enable stricter validation of internal invariants, such as the MEM structure’s enc field. The JDBC driver may handle string encodings differently than the CLI—for example, in how it binds parameters or processes function results—leading to mismatches that the CLI’s release build ignores.

4. Empty String and Implicit Type Conversion Edge Cases
   The empty string '' in SQLite can be interpreted as a zero-length BLOB or TEXT value depending on context. When used in a numeric function like ABS, it is converted to 0, but the encoding metadata for the resulting value may not be reset correctly, especially in debug builds.

Diagnosis, Workarounds, and Permanent Fixes

Step 1: Validate Database Encoding and Query Plan

• Check Effective Encoding:
  Run PRAGMA encoding; after database creation to confirm it is UTF-16le or UTF-16be. Some JDBC drivers override or misinterpret encoding pragmas.

• Analyze Query Plan Differences:
  Use EXPLAIN QUERY PLAN in both the CLI and JDBC driver to compare index usage. If the CLI avoids the index (e.g., due to different statistics), it may sidestep the encoding check.

Step 2: Resolve Type and Encoding Conflicts in the Query

• Explicit Type Casting:
  Modify the BETWEEN clause to enforce consistent types:

  SELECT * FROM t0 WHERE t0.c1 BETWEEN CAST('' AS TEXT) AND CAST(ABS('') AS TEXT);
  

  This ensures both sides of BETWEEN are treated as TEXT with matching encoding.

• Avoid Ambiguous Empty Strings:
  Replace ABS('') with an explicit zero:

  SELECT * FROM t0 WHERE t0.c1 BETWEEN '' AND 0;
  

  This forces SQLite to handle the comparison as text-to-numeric, which may bypass the debug assertion.

Step 3: Adjust JDBC Driver Configuration

• Disable Debug Assertions:
  Recompile the JDBC driver without -DSQLITE_DEBUG=1. This is not a fix but a mitigation for testing.

• Update SQLite Version:
  Ensure the JDBC driver uses SQLite 3.41.2 or newer. Older versions have known issues with encoding checks in debug builds.

Step 4: Modify Schema and Indexing Strategy

• Use COLLATE for Index Consistency:
  Define the index with a BINARY collation to suppress encoding-specific comparisons:

  CREATE INDEX i0 ON t0(c1 COLLATE BINARY);
  

• Store Numeric Values in Separate Columns:
  If c1 contains numeric data, store it in a REAL or INTEGER column to avoid implicit conversions.

Step 5: Patch or Custom Build SQLite

For advanced users:

• Modify valueFromFunction in SQLite Source:
  In sqlite3.c, locate the assertion and add a conditional to reset pVal->enc if db->mallocFailed is true. Example:

  assert(enc==pVal->enc || (pVal->flags & MEM_Str)==0 || db->mallocFailed);
  if (enc != pVal->enc && (pVal->flags & MEM_Str) != 0 && !db->mallocFailed) {
      sqlite3VdbeChangeEncoding(pVal, enc);
  }
  

  Warning: This requires deep knowledge of SQLite internals and may introduce instability.

Conclusion

The assertion failure stems from a combination of strict debug-mode checks, implicit type conversions, and JDBC-specific handling of UTF-16 encoded text. Solutions range from query adjustments and schema changes to driver reconfiguration. For production environments, using the official SQLite release build in the JDBC driver (without -DSQLITE_DEBUG=1) is recommended unless debugging is explicitly required. Developers should audit queries involving mixed-type comparisons in UTF-16 databases to preempt similar issues.