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Jack Ye
8f37234a86
Feature engineering is now the overview plus one page per operation: add a computed column, refresh a column, refresh a materialized view. Computed columns covered declaring and filling on one page, which are separate decisions with separate failure modes. Functions move to Management, where the other catalog objects are, and absorb the environment-mismatch guidance from the Enterprise page -- a symptom table worth having beside the parameters that cause it. The Enterprise feature-engineering page is merged into the overview. Two of its claims were stale: create_materialized_view is implemented for db:// connections in remote/db.py, so views are not local-only, and the tier is Enterprise rather than 'Cloud and Enterprise'. The REST reference leaves the sidebar. Mintlify only generates per-endpoint pages from a navigation group -- hidden, dropdowns, global anchors and a top-level openapi key were all tried -- so removing the group removes those pages. The overview survives as an unlisted page, linked from SDKs & SQL, and the spec still ships for anything that wants to read it directly.
90 lines
3.1 KiB
Plaintext
90 lines
3.1 KiB
Plaintext
---
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title: "Add a computed column"
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sidebarTitle: "Add Computed Column"
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description: "Declare a column from a SQL expression. Nothing is evaluated until you refresh it."
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icon: table-columns
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keywords: ["computed columns", "add_columns", "transforms", "schema"]
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---
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A computed column is declared with an expression and starts with no values.
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Declaring it is cheap on any table size, because nothing is evaluated until
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[a refresh](/feature-engineering/refresh-column) asks for it.
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## Declaring one {#declare}
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```python
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import lancedb
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db = lancedb.connect("./.lancedb")
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table = db.create_table("computed_demo", [{"x": 1}, {"x": 2}])
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table.add_columns(computed={"doubled": "x * 2"})
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# AddColumnsResult(version=2)
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```
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That commits a new table version with the column present and empty. It is an
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ordinary table version, so the usual [versioning](/tables/versioning) applies --
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including that you can travel back to before the column existed.
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## `computed` and `transforms` are different {#computed-vs-transforms}
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`add_columns` takes either, and the difference matters:
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- **`transforms`** evaluates the expression *now*, for every row, as part of the
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call. Good for a small table, or a column you want populated immediately.
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- **`computed`** stores the expression and evaluates nothing. The column is
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committed empty and filled later by
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[`refresh_column`](/feature-engineering/refresh-column).
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They cannot be combined in one call. Reach for `computed` when the table is
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large enough that you would rather control when the work happens.
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## The expression {#expressions}
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The expression is SQL, evaluated over the row. It can reference any column in
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the table, including another computed column:
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```python
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table.add_columns(computed={
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"power_score": "(stats.magic + stats.strength) / 2.0",
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"is_strong": "power_score > 3.0",
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})
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```
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Declaration order is dependency order -- a computed column may reference one
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declared earlier in the same mapping, but not one declared after it.
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The functions available are the ones the
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[SQL dialect](/sql/functions) documents.
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## Its inputs become load-bearing {#input-constraints}
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<Warning>
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While a computed column's declaration references another column, that column
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cannot be renamed, retyped, or dropped. The declaration is part of the schema,
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and LanceDB will not let you invalidate it out from under itself.
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</Warning>
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This is worth knowing before deriving a column from something you expect to
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reshape later. Dropping the computed column releases the constraint.
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## Filling with Python instead of SQL {#functions}
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<Badge color="red">Enterprise</Badge>
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When the value cannot be expressed in SQL -- an embedding, a model call, an
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image decode -- the column is filled by a registered
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[function](/management/function) instead of an expression. Declaring it looks
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the same; what differs is what fills it.
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## What to read next {#next}
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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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Fill the rows that lack values.
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</Card>
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<Card title="Functions" icon="code" href="/management/function">
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Fill a column with Python.
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</Card>
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</CardGroup>
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