EXPLAIN Plan Reader

Demo

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---
name: EXPLAIN Plan Reader
description: Read a captured EXPLAIN ANALYZE plan, name the single bottleneck node, and prescribe a targeted fix for it. Use when a specific query is slow and you have (or can capture) its execution plan from Postgres or MySQL and need to know WHICH node is burning the time and why.
---
# EXPLAIN Plan Reader

Turn one captured execution plan into a named bottleneck node and a specific fix. A plan is a tree executed bottom-up and inside-out; your job is to find the one node where time or rows explode, not to audit every line.

## Workflow

1. **Demand a real plan first.** Do not diagnose from query text alone. In Postgres, require `EXPLAIN (ANALYZE, BUFFERS, FORMAT TEXT)` — ANALYZE runs the query for real timings, BUFFERS shows cache-vs-disk reads. In MySQL, require `EXPLAIN ANALYZE`. Plain `EXPLAIN` shows estimates only and lies under data skew; if that is all you have, say so and ask for ANALYZE output before prescribing.
2. **Run it twice.** The first run can be cold cache. Trust the warm run's timings unless cold-cache I/O is itself the problem (confirm via BUFFERS: high `read=` vs `hit=`).
3. **Find the bottleneck node.** Postgres reports cumulative `actual time`; a node's own cost is its time minus its children's. Locate the node with the largest delta or the largest `loops`. `(actual rows=R loops=N)` means the node ran N times — multiply R×N for true rows. A Nested Loop with high `loops` driving a Seq Scan is the classic disaster. In MySQL, scan for `type=ALL` (full scan), large `rows` examined, and `Extra: Using filesort` / `Using temporary`.
4. **Confirm with the estimate gap.** Compare estimated vs actual rows on the hot node. A large gap (e.g. estimated 1, actual 500000) means the planner is flying blind — that misestimate usually causes the bad node choice upstream and is the root cause, not the node itself.
5. **Diagnose by node type and prescribe one fix:**
   - Seq Scan on a large table filtered to few rows → the predicate is not served by an index. Name the column(s) and predicate shape; check whether the WHERE wraps the column in a function (defeats a plain index). Hand the access pattern to index-advisor for the actual index choice.
   - Bad row estimate → stale or insufficient statistics. Run `ANALYZE the_table`; for skewed columns raise `default_statistics_target` and re-ANALYZE.
   - External merge / disk-spilling Sort → raise `work_mem` for the session, or provide pre-sorted input via an index ordering.
   - Hash Join with `Batches > 1` (spilling to disk) → raise `work_mem`.
   - Nested Loop chosen for a large set → almost always a row underestimate upstream; fix the estimate (step 4) and the planner switches to Hash/Merge Join on its own.
   - MySQL `Using filesort` with large `rows` → unindexed ORDER BY; provide an index covering the sort order.
6. **State the verdict in one line:** the node, its share of total time, the root cause, and the single highest-leverage fix.

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