Taxonomy: Fix WP_Term_Query cache miss for equivalent queries

[sanketio]

Summary

Fixes a duplicate database query that occurs on the Posts list table (and wherever wp_dropdown_categories() and wp_terms_checklist() are rendered on the same page) due to two semantically identical get_terms() calls producing different cache keys.

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Problem

WP_Term_Query::generate_cache_key() hashes the full set of sanitised query args together with the SQL:

$key = md5( serialize( $cache_args ) . $sql );

wp_dropdown_categories() passes hierarchical => 1 (int), while wp_terms_checklist() passes get => 'all' which parse_query() immediately normalises to hierarchical => false (bool). Both produce identical SQL, yet serialize() produces two different strings → two different cache keys → two identical DB queries on every page load.

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Solution

Replace the wide arg-serialisation approach with SQL + only the args that drive PHP-level post-processing (which is not captured by the SQL):

Arg	Reason kept
child_of	_get_term_children() runs in PHP; not always reflected in SQL
pad_counts	_pad_term_counts() runs in PHP and changes the cached data shape
prune_empty_terms	Combined (bool)($hierarchical && $hide_empty) — only the conjunction determines PHP pruning
number / offset (hierarchical only)	No LIMIT in SQL for hierarchical queries; PHP slices with array_slice()
fields	Preserved from existing logic; determines cache value shape

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Changes

• src/wp-includes/class-wp-term-query.php — Rewrites generate_cache_key() to key on SQL + a minimal set of PHP-post-processing args.
• tests/phpunit/tests/term/query.php — Adds four new @ticket 64038 / @group cache test methods covering: shared cache entry for equivalent queries, and separate keys for differing prune_empty_terms, child_of, and pad_counts.

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Testing

vendor/bin/phpunit tests/phpunit/tests/term/query.php --filter cache

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Trac ticket: https://core.trac.wordpress.org/ticket/64038

Use of AI Tools

AI assistance: Yes
Tool(s): GitHub Copilot (Claude Sonnet 4.6)
Used for: The problem analysis, approach selection, implementation, and test cases were reviewed and verified by the human author before submission. All code changes have been manually checked for correctness against the WordPress codebase.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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