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ClaudSkills / Engineering / backend

Multixact

Category: Engineering  ·  Sub-category: backend  ·  Last updated:
PostgreSQL's MultiXact machinery — `src/backend/access/transam/multixact.c` — the multi-transaction ID system that lets a single heap tuple record multiple concurrent lockers (for `SELECT ... FOR SHARE` and `SELECT ... FOR UPDATE` blends) plus the multixact-freeze / wraparound path. Loads when the user asks about MultiXactId semantics, `xmax` interpretation with the `HEAP_XMAX_IS_MULTI` bit, why a tuple can appear to be locked by multiple transactions, tuple-locking modes (`FOR SHARE` / `FOR NO KEY UPDATE` / `FOR KEY SHARE`), the multixact SLRU (`pg_multixact/`), MultiXact member offsets, or the `autovacuum_multixact_freeze_max_age` wraparound path. Skip when the ask is about clog (regular transaction commit status — sibling but different code) or 2PC prepared transactions.
Security AStatic scan found no risk patternsHow grading works ›

From the source SKILL.md

Every heap tuple has an xmax field. In the simple case, xmax is the XID of a transaction that either deleted or exclusively locked the tuple. But PG supports multiple concurrent lockers on the same tuple (row-level shared locks + key-share locks + non-blocking-with-updates), which requires representing "this tuple is currently locked by transactions A, B, and C" in a fixed-size field.

What this skill does

Multixact is a community-contributed Claude Code skill in the backend sub-category. It ships as a SKILL.md file that Claude Code auto-discovers under ~/.claude/skills/multixact/ and loads when your prompt matches the skill's trigger.

Who uses this skill

The Multixact Claude Code skill is built for software engineers, backend developers, full-stack teams, and technical leads building and maintaining production systems. It's part of ClaudSkills (also referred to as Claude Skills or Claude Code Skills) — the open community-curated registry of 158,000+ SKILL.md files for Anthropic's Claude Code agent and the wider Claude ecosystem (Claude API, Claude Agent SDK).

How to install

Free

Manual install (2 steps)

mkdir -p ~/.claude/skills/multixact
curl -L https://claudskills.com/skills/multixact/SKILL.md \
  -o ~/.claude/skills/multixact/SKILL.md

Or just download SKILL.md directly and drop it into ~/.claude/skills/multixact/. Claude Code auto-discovers it on next session.

Skills live at ~/.claude/skills/multixact/SKILL.md on macOS/Linux, or %USERPROFILE%\.claude\skills\multixact\SKILL.md on Windows. See the full install guide for step-by-step instructions.

Telegram

📱 Install from your phone or desktop Telegram

Open @claudskills_bot on Telegram, tap Open Desktop App, and the desktop app installs this skill for you. Or share the bot link with a colleague — they get the same one-tap install. Learn more →

Pro

One-click install via the desktop app

The ClaudSkills desktop app installs any skill directly into ~/.claude/skills/ with one click — no terminal required. Pro starts at $9/mo or $149 lifetime.

Pro

For the full experience including quality scoring and one-click install features for each skill — upgrade to Pro.

See pricing → Download desktop app

Frequently asked questions

How do I install the Multixact Claude Code skill?
Install via the ClaudSkills desktop app (one click) or copy SKILL.md from the source repository to ~/.claude/skills/multixact/SKILL.md and restart Claude Code. Both flows are detailed at claudskills.com/install/.
What does the Multixact skill do?
PostgreSQL's MultiXact machinery — `src/backend/access/transam/multixact.c` — the multi-transaction ID system that lets a single heap tuple record multiple concurrent lockers (for `SELECT ... FOR SHARE` and `SELECT ... FOR UPDATE` blends) plus the multixact-freeze / wraparound path. Loads when the user asks about MultiXactId semantics, `xmax` interpretation with the `HEAP_XMAX_IS_MULTI` bit, why a tuple can appear to be locked by multiple transactions, tuple-locking modes (`FOR SHARE` / `FOR NO KEY UPDATE` / `FOR KEY SHARE`), the multixact SLRU (`pg_multixact/`), MultiXact member offsets, or the `autovacuum_multixact_freeze_max_age` wraparound path. Skip when the ask is about clog (regular transaction commit status — sibling but different code) or 2PC prepared transactions.
Is this skill free to install?
Yes. ClaudSkills is an open registry — every skill keeps its source repository's license, and manual install via copy is free. ClaudSkills Pro ($9/mo, $79/yr, or $149 one-time) adds one-click install via the desktop app and a multi-signal Quality Score.
When should I use the Multixact skill?
Use Multixact when your Claude Code task falls under the Engineering category — specifically in the backend area. Claude Code auto-discovers installed skills and invokes the right one based on the task description, so you can also ask Claude directly (e.g. "use Multixact" or describe the task and let Claude pick). Browse related skills at /category/engineering/.
What is a Claude Code skill and how does the Multixact skill fit in?
A Claude Code skill is a SKILL.md file that lives under ~/.claude/skills/<name>/ and tells the Claude Code CLI agent how to perform a specific task (instructions, prompts, allowed tools). Skills are auto-discovered at session start. Multixact is one of 67,000+ skills indexed in the open ClaudSkills catalog, classified under the Engineering category. Learn more at /learn/what-is-a-claude-skill/.

Attribution & license

Cite this skill

If you reference this skill in a blog post, paper, or documentation, you can cite it as:

APA
matejformanek. (2026). Multixact [Claude Code skill]. ClaudSkills. https://claudskills.com/skills/multixact/
BibTeX
@misc{multixact-2026,
  author    = {matejformanek},
  title     = {Multixact [Claude Code skill]},
  year      = {2026},
  publisher = {ClaudSkills},
  url       = {https://claudskills.com/skills/multixact/}
}

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Grade A · scanned 2026-07-23 — free static scan against the OWASP Agentic Skills Top 10.

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