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BigQuery bills you for bytes read, not rows returned. A practical guide to partition pruning and the filters that silently defeat it, clustering, pushing filters below joins, pre-aggregation, and finding your real spend in INFORMATION_SCHEMA.
Why iterrows is slow, what actually happens when you cross from a NumPy array into Python objects, and how to replace row loops with boolean masks, np.select, map, merge and groupby transform — plus the cases where a plain loop is still the right answer.
Stop guessing at slow SQL. A practical guide to EXPLAIN ANALYZE: how to read the plan tree, the four numbers that actually matter, why per-loop timings mislead, and why bad row estimates cause more slow queries than missing indexes.