An index plan that is perfect at a million rows becomes a liability at a billion. We walk one events table through four sizes, show where the planner changes its mind, and what we added, removed or replaced at each stage.
A shipment timeline query had a compound index yet still took 2.4 seconds. explain() showed a blocking SORT stage and 180,000 documents examined for 50 results. Reordering the index by Equality, Sort, Range fixed it.
EXPLAIN ANALYZE prints a wall of nodes, costs and loops that most of us skim for the word Seq Scan. This guide walks a real slow query node by node: estimates vs actuals, loops, buffers, and the fixes each pattern points to.
Our invoice list took 3 seconds even though a composite index covered both filter columns. The range column was first, and two other queries wrapped the column in a function or a cast. Reordering and rewriting fixed all three.