Optimize SnapBuildPurgeOlderTxn: use in-place compaction instead of temporary array

Xuneng Zhou <xunengzhou@gmail.com>

From: Xuneng Zhou <xunengzhou@gmail.com>

To: pgsql-hackers <pgsql-hackers@lists.postgresql.org>

Date: 2025-10-18T07:49:40Z

Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

Introduce logical decoding.
- b89e151054a0 9.4.0 cited

Attachments

v1-0001-Optimize-SnapBuildPurgeOlderTxn-function.patch (application/octet-stream) patch v1-0001

Hi Hackers,

The SnapBuildPurgeOlderTxn function previously used a suboptimal
method to remove old XIDs from the committed.xip array. It allocated a
temporary workspace array, copied the surviving elements into it, and
then copied them back, incurring unnecessary memory allocation and
multiple data copies.

This patch refactors the logic to use a standard two-pointer, in-place
compaction algorithm. The new approach filters the array in a single
pass with no extra memory allocation, improving both CPU and memory
efficiency.

No behavioral changes are expected. This resolves a TODO comment
expecting a more efficient algorithm.

Best,
Xuneng