Assertion Failure in sqlite3VdbeExec Due to Cursor Initialization in Complex Query

Understanding the Core Failure: Cursor Initialization in Window Function Contexts

The assertion failure pC!=0 in sqlite3VdbeExec arises when SQLite attempts to access a database cursor (VdbeCursor) that has not been properly initialized or has been prematurely closed during query execution. This occurs specifically in queries involving window functions, aggregation with GROUP BY, and collation-sensitive views when certain query optimizations are enabled. The failure is triggered by a mismatch between the query planner’s assumptions about cursor availability and the actual state of cursor lifecycle management during the execution of window functions in HAVING clauses.

The error manifests in the following scenario:

1. A table (v0) with a UNIQUE column (c1) and a second column (c) is created.
2. An INSERT operation populates only column c, leaving c1 as NULL.
3. A view (v1) is defined to select c1 with COLLATE NOCASE, which implicitly casts NULL values into a collation context.
4. A SELECT query performs a LEFT JOIN between v0 and v1, groups the results using GROUP BY 1, and applies a HAVING clause containing a subquery with substr() and lag() window functions.

The assertion failure occurs because the cursor (pC) associated with the window function’s partition or the underlying view’s collation processing is not initialized when the lag() function attempts to reference it. This is exacerbated by query optimizations (e.g., SQLITE_CoverIdxScan) that alter cursor lifecycle management.

Root Causes: Query Optimization, Collation, and Window Function Interactions

1. Incorrect Cursor Lifecycle Assumptions in Optimization Flags

The SQLITE_CoverIdxScan optimization (enabled by default) allows SQLite to use covering indices to avoid table lookups. However, when this optimization interacts with queries involving window functions and collation rules, it may prematurely close cursors or skip their initialization. The lag() window function in the HAVING clause’s subquery requires a cursor to traverse partitioned data, but if the optimizer assumes the cursor is unnecessary (due to covering index logic), pC remains uninitialized, triggering the assertion.

2. Collation Rules and Implicit NULL Handling in Views

The view v1 applies COLLATE NOCASE to c1, which modifies how NULL values are handled during comparisons. Since the INSERT into v0 leaves c1 as NULL, the collation rule forces SQLite to treat NULL as a valid value in the view’s output. This collation context propagates to the LEFT JOIN and GROUP BY operations, creating a dependency on cursor states that are not properly managed when the query includes window functions.

3. Window Function Execution During HAVING Clause Evaluation

The HAVING clause is evaluated after GROUP BY, meaning the subquery containing lag() must process aggregated data. Window functions like lag() rely on cursors to iterate over partitions, but if the query planner fails to allocate a cursor for the partition (due to PARTITION BY 0, which groups all rows into a single partition), the cursor (pC) is not created, leading to the assertion failure. The PARTITION BY 0 clause is particularly problematic because it creates a degenerate partition that may bypass cursor initialization logic.

Resolution: Debugging, Workarounds, and Code Fixes

Step 1: Diagnose Query Execution with EXPLAIN and Optimization Control

Begin by analyzing the query execution plan using EXPLAIN and EXPLAIN QUERY PLAN. Compare the output with and without the SQLITE_CoverIdxScan optimization:

.testctrl optimizations 0x00000020  -- Disable CoverIdxScan
EXPLAIN QUERY PLAN
SELECT 0 FROM v0 LEFT JOIN v1 AS a0 GROUP BY 1 HAVING ...;

Observe whether disabling the optimization changes the use of cursors for the window function or the view’s collation processing. If the query succeeds with the optimization disabled, this confirms that cursor management under CoverIdxScan is flawed.

Step 2: Modify the Query to Isolate the Issue

Temporarily simplify the query to identify the exact component causing the failure:

• Remove the COLLATE NOCASE from the view definition. If the assertion no longer occurs, the collation rule is contributing to cursor mismanagement.
• Replace the lag(0) window function with a constant. If the error disappears, the window function’s cursor requirements are the culprit.
• Populate c1 with non-NULL values in v0. If the query succeeds, the interaction between NULL handling and cursor initialization is faulty.

Step 3: Patch the Cursor Initialization Logic

The root cause lies in the sqlite3VdbeExec function’s handling of cursors for window functions in degenerate partitions. Modify the code to ensure cursors are initialized even for PARTITION BY 0 clauses:

1. In sqlite3WindowCodeStep(), add a check for empty or constant partition expressions. Force the allocation of a cursor for these cases.
2. In the optimization logic for CoverIdxScan, add a condition to skip the optimization if the query contains window functions with degenerate partitions.

Step 4: Apply Compilation Flags for Debugging

Recompile SQLite with debugging flags to trace cursor activity:

export CFLAGS="-g -O0 -DSQLITE_DEBUG -DSQLITE_ENABLE_TREETRACE -DSQLITE_ENABLE_WHERETRACE"
./configure
make

Run the query with tracing enabled:

.tree
.trace
SELECT 0 FROM v0 LEFT JOIN v1 AS a0 GROUP BY 1 HAVING ...;

Inspect the logs for cursor initialization steps and identify where pC is not assigned.

Step 5: Implement Runtime Workarounds

If patching SQLite is not feasible, use these runtime workarounds:

• Disable SQLITE_CoverIdxScan with .testctrl optimizations 0x00000020 before executing the query.
• Rewrite the query to avoid PARTITION BY 0 and lag(0). For example, use PARTITION BY c1 and handle NULLs explicitly.
• Materialize the view v1 into a temporary table to bypass collation-related cursor issues:

CREATE TEMP TABLE temp_v1 AS SELECT c1 COLLATE NOCASE FROM v0;
SELECT 0 FROM v0 LEFT JOIN temp_v1 AS a0 ...;

Step 6: Validate with Regression Tests

After applying code fixes, run regression tests to ensure the assertion failure does not recur. Include test cases for:

• Queries with PARTITION BY constant values.
• Views using COLLATE clauses on columns with NULL values.
• HAVING clauses containing subqueries with window functions.

By systematically addressing cursor lifecycle management in window function execution and collation processing, this assertion failure can be resolved.