Performance: optimize subscriber database queries

[danieliser]

Scope

Standalone performance change against develop for subscriber database reads and WordPress privacy lookups.

TL;DR

Moves subscriber pagination totals from full-row hydration to COUNT(*), and routes WordPress privacy export/erase through the existing indexed email column rather than a broad substring scan across every subscriber field.

Measurement method

MySQL 8.0.35 / PHP 8.3, 100,000-row temporary copy of the real pum_subscribers schema and indexes. Temporary tables were session-scoped and left the site database unchanged. Timing used 12 warm alternating samples; memory used isolated processes so the full-row baseline did not contaminate the count measurement.

Operation	Before	After	Gain
Pagination count retained PHP memory	195,016,736 B	16,472 B	-195,000,264 B (-99.99%)
Pagination count elapsed (isolated sample)	199.45 ms	128.50 ms	-35.6%
Privacy lookup median	225.95 ms	0.20 ms	~1,159x faster
Privacy lookup query plan	full scan, ~100,102 rows	email index, 1 row	indexed point lookup

The count timing is storage-engine dependent; the primary deterministic gain is that PHP no longer transfers, allocates, and retains every matching row merely to count them.

Compatibility

• Free-text subscriber search is unchanged and still searches the selected fields.
• Pagination, ordering, selected fields, and return formats are unchanged.
• Privacy export/erase already accepted only strict email matches after querying; the database now applies that same exact predicate first.
• Exact filters are limited to declared subscriber columns and use their existing prepared value formats.

Validation

• Full GitHub Tests matrix and CI - Code Quality: green on the final SHA.
• Focused subscriber suite: 59 tests, 128 assertions.
• Changed production-file PHPCS at CI error severity: passed.
• PHP syntax and git diff --check: passed.
• Original scoped commit replayed unchanged onto current develop (git range-diff exact match).

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📥 Commits

Reviewing files that changed from the base of the PR and between 876b6de and 6716233.

📒 Files selected for processing (3)

• classes/DB/Subscribers.php
• classes/Privacy.php
• tests/php/tests/PUM_DB_Subscribers_Test.php

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[danieliser]

@codex review

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Codex Review: Didn't find any major issues. Keep them coming!

Reviewed commit: 451693e5b1

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[danieliser]

@codex review

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Codex Review: Didn't find any major issues. More of your lovely PRs please.

Reviewed commit: 5efe5f55f4

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Codex Review: Didn't find any major issues. 🎉

Reviewed commit: 6716233cd8

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