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Brendan Clement
6367f7a0de
Five pages still said a refresh never revisits a filled row and that a changed input needs the column dropped and declared again. Since lancedb#4161 a refresh also recomputes rows whose inputs changed, so the quickstart, the feature-engineering overview, Guarantees, and the SQL REFRESH and UPDATE pages now say that. Retry claims are qualified the same way. A local refresh commits the column once, and a deployment's job commits finished batches. A retry fills what is still empty or stale, but can recompute values that were already written (a refresh whose input record was not saved recomputes next time), so no page promises that a retry costs only the unfinished rows.
97 lines
3.1 KiB
Plaintext
97 lines
3.1 KiB
Plaintext
---
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title: "REFRESH"
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sidebarTitle: "REFRESH"
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description: "Filling computed columns and bringing materialized views up to date."
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icon: arrows-rotate
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keywords: ["refresh column", "refresh materialized view", "backfill", "cascade"]
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---
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<Badge color="red">Enterprise</Badge> [SQL runs on a deployment.](/sql)
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LanceDB extensions with no DataFusion equivalent. They are what makes
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[feature engineering](/feature-engineering) reachable from SQL: a column
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declared by [DDL](/sql/ddl/table#computed-column) starts empty, and a refresh fills it.
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## REFRESH COLUMN {#refresh-column}
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```sql
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REFRESH COLUMN <column> ON [TABLE] <table>
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[WHERE <predicate>]
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[WITH (<option> = <value> [, ...])]
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```
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```sql
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REFRESH COLUMN doubled ON my_table
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REFRESH COLUMN embedding ON documents WHERE created_at > '2026-01-01'
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```
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A refresh gives values to rows that lack them, recomputes rows whose inputs
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changed, and **leaves other filled rows alone**. That makes it idempotent, cheap
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to repeat as data arrives, and safe to re-run after a failure -- though a re-run
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can recompute values the failed run had already written.
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`WHERE` scopes the refresh, which is how you fill a slice and check it before
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committing to the whole table.
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### Options {#refresh-column-options}
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| Option | Meaning |
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| --- | --- |
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| `num_workers` | how many workers to use |
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| `cpu_millis` | CPU budget per worker |
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| `memory_bytes` | memory budget per worker |
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| `gpu_count` | GPUs per worker |
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| `batch_size` | rows a worker takes at a time |
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| `error_policy` | `'skip'` continues past failing rows instead of failing the job |
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```sql
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REFRESH COLUMN embedding ON documents
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WITH (cpu_millis = 8000, memory_bytes = 17179869184, gpu_count = 2)
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```
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`num_workers` sets a count; the budget options size each worker. They are
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alternatives rather than a pair.
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<Note>
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`error_policy` and `num_workers` are refused for a column filled by a registered
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[function](/management/function) with its own resource budget -- use the CPU,
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memory and GPU options there.
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</Note>
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## REFRESH MATERIALIZED VIEW {#refresh-materialized-view}
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```sql
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REFRESH MATERIALIZED VIEW <name> [WITH (<option> = <value>)]
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```
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```sql
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REFRESH MATERIALIZED VIEW chunks
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REFRESH MATERIALIZED VIEW chunks WITH (full = true)
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```
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By default a refresh is **incremental** -- it processes what the source has
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added since the last one. `full = true` recomputes every row, which is what you
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want after changing something the view's rows depend on.
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<Warning>
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`CASCADE` parses but is **not implemented**. A view built on another view is
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refreshed by refreshing each in dependency order yourself.
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</Warning>
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## Watching a refresh {#watching}
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Both statements submit work that outlives the call. Track it with
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[`SHOW JOBS`](/sql/ddl/job#show), and diagnose failures with
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[`SHOW ERRORS`](/sql/ddl/job#errors).
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## See also {#see-also}
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<CardGroup cols={2}>
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<Card title="Refresh a column" icon="arrows-rotate" href="/feature-engineering/refresh-column">
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The same operation from the SDKs, and what a refresh guarantees.
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</Card>
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<Card title="Table" icon="table" href="/sql/ddl/table#alter">
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Declaring the columns this fills.
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</Card>
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</CardGroup>
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