Function execute_backfill 

pub async fn execute_backfill(
    db: &Database,
    step: &ExecutionStep,
    step_index: usize,
) -> Result<BackfillResult, MigrationError>

Execute a backfill ExecutionStep against db, deriving and returning matched/inserted/skipped counts.

Execution sequence

1. Run a Read count query over the guarded match clause (the insert-if-absent predicate — rows that will be inserted).
2. Run a Read count query over the unguarded match clause (all candidate rows, including those already having the destination).
3. Run the forward TypeQL under a Write transaction and commit.

Steps 1 and 2 run before the write so the counts reflect the pre-write state.

Invariant 2

Both count queries are derived from step.forward by string manipulation of the carried match clause. No OperationSpec is inspected here.