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Aggregation function lastChangeAt#103458
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Returns the `calculated_at` timestamp of the most recent actual change in `value` across a time-ordered sequence. A change is counted only when the value genuinely differs between consecutive observations. If the value never changes, the first `calculated_at` is returned. Supported value types: `(U)Int*`, `Float*`, `Decimal*`, `Date`, `DateTime`, `DateTime64`, and `AggregateFunction(groupBitmap, UInt*)`. Supported timestamp types: `(U)Int*`, `Float*`, `Date`, `DateTime`, `DateTime64`. Designed for `AggregatingMergeTree` to maintain a `changed_at` column without storing full history. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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Changelog category: New Feature
Changelog entry:
Added aggregate function
lastChangeAt(value, calculated_at)that returns the time of the most recent actual change invalueacross a time-ordered sequence. Supports numeric, Decimal, Date, DateTime, DateTime64, and BitmapData value types. Designed for use with AggregatingMergeTree to maintain achanged_atcolumn without storing full history.Documentation entry:
Motivation:
In AggregatingMergeTree tables it is sometimes necessary to track when a metric last changed its value. Storing the full history is expensive, and without lastChangeAt there is no way to answer "when did this value last change?" across incremental merges while keeping only a fixed-size state.
Parameters:
value— the metric to observe. Supported types:(U)Int*,Float*,Decimal*,Date,DateTime,DateTime64,AggregateFunction(groupBitmap, UInt*). For bitmaps, equality is tested viabitmapXorCardinality.calculated_at— timestamp of each observation (rows must be ordered by this column). Supported types:(U)Int*,Float*,Date,DateTime,DateTime64.Example: