> For the complete documentation index, see [llms.txt](https://docs.postgresql.tw/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.postgresql.tw/internals/transactions/transaction-id.md).

# 67.1. Transactions and Identifiers \#

Transactions can be created explicitly using `BEGIN` or `START TRANSACTION` and ended using `COMMIT` or `ROLLBACK`. SQL statements outside of explicit transactions automatically use single-statement transactions.

Every transaction is identified by a unique `VirtualTransactionId` (also called `virtualXID` or `vxid`), which is comprised of a backend's process number (or `procNumber`) and a sequentially-assigned number local to each backend, known as `localXID`. For example, the virtual transaction ID `4/12532` has a `procNumber` of `4` and a `localXID` of `12532`.

Non-virtual `TransactionId`s (or `xid`), e.g., `278394`, are assigned sequentially to transactions from a global counter used by all databases within the PostgreSQL cluster. This assignment happens when a transaction first writes to the database. This means lower-numbered xids started writing before higher-numbered xids. Note that the order in which transactions perform their first database write might be different from the order in which the transactions started, particularly if the transaction started with statements that only performed database reads.

The internal transaction ID type `xid` is 32 bits wide and [wraps around](/server-administration/maintenance/routine-vacuuming.md#VACUUM-FOR-WRAPAROUND) every 4 billion transactions. A 32-bit epoch is incremented during each wraparound. There is also a 64-bit type `xid8` which includes this epoch and therefore does not wrap around during the life of an installation; it can be converted to xid by casting. The functions in [Table 9.84](/the-sql-language/functions/functions-info.md#FUNCTIONS-PG-SNAPSHOT) return `xid8` values. Xids are used as the basis for PostgreSQL's [MVCC](/the-sql-language/mvcc.md) concurrency mechanism and streaming replication.

When a top-level transaction with a (non-virtual) xid commits, it is marked as committed in the `pg_xact` directory. Additional information is recorded in the `pg_commit_ts` directory if [track\_commit\_timestamp](/server-administration/runtime-config/runtime-config-replication.md#GUC-TRACK-COMMIT-TIMESTAMP) is enabled.

In addition to `vxid` and `xid`, prepared transactions are also assigned Global Transaction Identifiers (GID). GIDs are string literals up to 200 bytes long, which must be unique amongst other currently prepared transactions. The mapping of GID to xid is shown in [`pg_prepared_xacts`](/internals/views/view-pg-prepared-xacts.md).

***

原文：[PostgreSQL 18.6 Documentation](https://www.postgresql.org/docs/18/transaction-id.html)（英文原文，待翻譯）


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://docs.postgresql.tw/internals/transactions/transaction-id.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.