How does Spanner's query optimizer choose an execution plan?

Spanner's optimizer is cost-based: it evaluates candidate plans using table and index statistics, such as row counts and cardinality estimates, and picks the plan with the lowest estimated cost in terms of data scanned and rows processed.

Key inputs to that decision include which indexes exist and whether they're covering for the query's columns, whether a join can be satisfied by co-located interleaved data instead of a distributed join, and predicate selectivity, which determines whether filtering early via an index beats scanning the base table. Developers can inspect the chosen plan with EXPLAIN or the query plan visualizer in the console, and can nudge the optimizer using index hints (FORCE_INDEX) when it consistently picks a suboptimal plan, though this should be a last resort after confirming statistics and indexes are actually appropriate for the query.

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