explain-joins: reword "create ... on disk" as "spill ... to disk"

[yahonda]

What is changed, added or deleted? (Required)

In the Hash Join section of explain-joins.md, one sentence describes the on-disk behavior as:

TiDB will attempt to use temporary storage, and might create the Build operator (used as part of the hash join) on disk.

"create the Build operator on disk" is confusing — the operator is not created on disk; its data is spilled to temporary storage. Later on the same page, the system-variables section already describes the same behavior with clearer wording:

TiDB will attempt to spill the Build operator of a hash join to disk to save memory.

This PR aligns the earlier sentence with that clearer, consistent wording ("spill ... to disk").

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)

What is the related PR or file link(s)?

N/A — documentation wording clarification.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Documentation
  • Clarified that when the query memory quota is exceeded and temporary storage on out-of-memory is enabled, the hash join Build operator may spill to disk.

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 5e0ed95 and cfcb41c.

📒 Files selected for processing (1)

• explain-joins.md

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📝 Walkthrough

Walkthrough

The Runtime Statistics documentation now states that TiDB may spill the hash join Build operator to disk when tidb_mem_quota_query is exceeded and temporary storage on OOM is enabled.

Changes

Runtime Statistics

Layer / File(s)	Summary
Hash join spill wording explain-joins.md	Clarifies that TiDB spills the hash join Build operator to disk instead of creating it on disk.

Estimated code review effort: 1 (Trivial) | ~2 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the doc wording change from "create ... on disk" to "spill ... to disk" in explain-joins.md.
Description check	✅ Passed	The description covers the required change summary, affected version, and related link section in the repository template.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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