docs: refresh stale EXPLAIN samples and fix invalid examples in the optimization and index pages

[ggevay]

Motivation

EXPLAIN has defaulted to the physical plan since #33448 (2025-06), but the optimization page still shows a bare EXPLAIN producing "Optimized Plan" MIR output. Separately, two CREATE INDEX examples fail if you run them, and the shared index-limitations note contradicts the same page. Everything below was checked against a v26.39.0 emulator or the code.

Description

transform-data/optimization.md — three plan samples regenerated verbatim from v26.39.0:

• The point-lookup sample showed MIR (ReadIndex on=… foo_x_y_idx=[lookup value=(42, 50)]). A bare EXPLAIN prints the fast-path arrow form, so the sample is now the real output (→Index Lookup on … / Lookup values: (42, 50)), and the surrounding prose no longer tells readers to look for lookup_value.

• The two delta-join samples are reachable only via EXPLAIN OPTIMIZED PLAN FOR, so the commands say that. The regenerated output adds what a release build actually prints: humanized column names (#0{id}), // { arity: N } annotations, and the Target cluster: footer. The <---------- Delta join teaching callout is kept.

  (Using OPTIMIZED PLAN rather than switching the samples to the physical plan is deliberate: a bare EXPLAIN over a SELECT renders One-Shot Delta Join, a one-shot form that the index or materialized view the section is about would not use.)

Index usage types — the list omitted three variants that IndexUsageType renders (src/repr/src/explain.rs): plan root (no new arrangement), sink export, index export. Added, and the lead-in now says "most common" rather than "all possible".

headless/index-ordering.md (rendered on this page and on concepts/indexes) said indexes provide no optimization for LIMIT, contradicting the fast path limit usage type documented a few lines above it. Now carves out that exception.

sql/create-index.md:

• "creates an in-memory index on a source, view, or materialized view" — the planner also accepts tables. Fixed in the page and in the two rendered syntax entries in data/examples/create_index.yml.
• The active_customers example used now(), which cannot be materialized (cannot materialize call to current_timestamp, with a hint pointing at mz_now()); rewritten as the temporal filter mz_now() < last_active_on + INTERVAL '30' DAYS.
• The "Speed up filtering" example had GROUP BY geo_id with two ungrouped columns, which fails to plan; the clause is dropped (the example only needs the columns).

sql/create-materialized-view.md — a duplicate include-from-yaml used the wrong parameter (file= instead of data=) and a name that does not exist, so it silently rendered nothing; the correct include is already a few lines above. Removed.

Retired links — ../../ops/optimization/, /overview/arrangements (×3), and the old Top K path now point at their current locations.

Verification

Prose and samples only. hugo builds cleanly. All three plan samples were captured from a v26.39.0 emulator using this page's own schema, and both failing CREATE INDEX examples were confirmed to error before the fix.