Individual skill

index-advisor

Recommends, orders, and prunes indexes for a specific query or table - composite column order, selectivity rules, partial and covering indexes, duplicate/unused cleanup, and write-amplification tradeoffs. Use when a query is slow and EXPLAIN shows a Seq Scan or a sort, before adding a CREATE INDEX, or when auditing a table's index set for bloat or duplicates. Do NOT use to diagnose an unknown slow query from scratch - start with sql-query-optimizer; do NOT use when the query shape itself is the problem (function-wrapped predicates, leading wildcards, correlated subqueries) - use query-rewriter instead; do NOT use when the table is too large and needs partitioning or a time-series strategy - use partition-planner instead.

skills/index-advisor/SKILL.md

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# Index Advisor

An index trades write speed and disk for read speed. Add the fewest indexes that serve real query shapes, order their columns deliberately, and prove every change with a plan. The costly mistake is the speculative index: it fails to help the read (wrong column order, predicate too unselective) while permanently taxing every write. This skill owns index design only - plan diagnosis belongs to sql-query-optimizer, and query restructuring belongs to query-rewriter.

## Inputs to collect

1. The slow query text and its `EXPLAIN (ANALYZE, BUFFERS)` output (Postgres) or `EXPLAIN ANALYZE` / `EXPLAIN FORMAT=JSON` (MySQL). No plan, no recommendation.
2. The table's row count and the existing index list (`\d table` / `SHOW INDEX`).
3. Predicate selectivity: roughly what fraction of rows each WHERE predicate matches. Estimate from `n_distinct` in `pg_stats` or a quick `COUNT(*)` ratio; label estimates as guesses.
4. The write profile: is this table write-hot (high INSERT/UPDATE rate), and which columns get updated?
5. For an audit: `pg_stat_user_indexes` (or the MySQL equivalent) over a representative window.

## Workflow