Search the library

Check the cardinality of each join. Joining one order to several lines produces multiple rows per order; joining another one-to-many table can multiply those rows again. Aggregate each relation to the intended grain before joining or use EXISTS when you only need to test presence. DISTINCT is not a reliable repair for incorrect join logic.

ForkJoinPool uses worker-local deques and lets idle workers steal tasks from others, which helps recursive, CPU-bound workloads with many small tasks. Long blocking I/O can occupy workers and reduce progress. Use a suitable dedicated executor or managed blocking strategy for blocking operations, and avoid creating huge numbers of tiny tasks with excessive scheduling overhead.

If each transaction sees a snapshot where another row satisfies a rule, they may update different rows and both commit, leaving the combined invariant false. Use a constraint, lock a shared coordination row, or use serializable isolation with whole-transaction retry. Exact Repeatable Read semantics differ by engine.

It narrows the investigation but does not identify one root cause: route failure, filtering, destination outage, asymmetric routing, or capture at the wrong interface can all look similar. Capture at both ends, check socket listeners and network policy, and correlate with route and firewall telemetry.

COUNT(*) counts each row in the group regardless of column values. COUNT(column) counts only rows where that expression is non-NULL. When aggregates look low, check nullability and join multiplication before changing the query.