多个事务同时提交问题

## Why you need it?
Is your feature request related to a problem? Please describe in details

## How it could be?
A clear and concise description of what you want to happen. You can explain more about input of the feature, and output of it.

## Other related information
Add any other context or screenshots about the feature request here.

The issue title raises a concurrent transaction-commit problem, but the body contains only the feature-request template and names no files, tests, entry points, or expected behavior. Start by locating the transaction commit flow and reproducing the simultaneous-commit case; the issue needs a defined failure scenario and acceptance criteria before work can be considered done.

索引モデルが issue の本文から書いたものです。