Live View, but tell me what changed

In 2019 ClickHouse got an experimental LIVE VIEW you could WATCH; it pushed a new result when the source table changed. The output carried a _version column but not what kind of change had happened. Issue #7881 asked for an _operation column next to it: +1 inserted, 0 updated, -1 deleted, so that dashboards, alerting and cache invalidators could react to changed or removed rows, not only new ones. The signed marker is the same row-and-cancellation model CollapsingMergeTree uses with sign.

Nothing happened, visibly. The issue has no comments; on 2022-04-26 it was closed without a word. GitHub records “completed”, but there is no PR and no _operation column ever appeared. Live View never left experimental status, which is the likely reason, though that is my inference.

The end of the story is documented: PR #88706, “Remove LIVE VIEW”, merged 2025-10-23, so the feature is gone in 25.11. The changelog warns that upgrading with a live view in place is not possible. WINDOW VIEW and WATCH were removed later, with streaming queries as the stated replacement direction (#114738).

On 26.4 the syntax no longer parses:

CREATE LIVE VIEW lv AS SELECT 1 FROM t;

Code: 62. DB::Exception: Syntax error: failed at position 13 (VIEW)

The need, “what changed since I last looked”, is still served by your own plumbing: a materialized view feeding a change table, or polling a monotonic column. If streaming queries land, a change feed with an operation marker is what I would ask for first.