SQL: wrapping an indexed column in a function disables the index

The index stores the column's values, not the function's results. Any expression around the column forces the engine to compute it per row, which means reading every row.

• SQL: a stale row estimate is why the planner chose the wrong join
• SQL: READ COMMITTED lets the same query return different rows twice
• SQL: consistent lock ordering is the only cure for deadlocks
• SQL: SELECT DISTINCT is usually a join fan-out you have not fixed
• SQL: COUNT(column) is not COUNT(*), and neither is COUNT(DISTINCT)
• SQL: OFFSET pagination gets slower the deeper the user goes

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Published 2026-08-25

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