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Brendan Clement
e20664740f
The two Function pages declare `launch: functions`, and every other page marks what it says about Functions, so the assembled site leaves them out until the launch. Before it, the pages that offered Functions for Python work point Enterprise readers at Geneva instead. The SQL Function example now uses Arrow type names, which the server requires, and states the 3.12 default for `python_version`.
117 lines
4.2 KiB
Plaintext
117 lines
4.2 KiB
Plaintext
---
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title: "Add a computed column"
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sidebarTitle: "Computed Columns"
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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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availability:
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oss: available
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enterprise: varies
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summary: >-
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Computed columns work in the embedded library. On a deployment they are off
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by default and have to be enabled there.
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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](/management/table#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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- **SQL expressions in `transforms`** are evaluated *now*, for every row, as
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part of the call. Good for a small table, or a column you want populated
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immediately.
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{/* if functions launched */}
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A `FunctionApplication` passed through `transforms` instead
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declares empty Function-bound columns, which a refresh fills; that needs a
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deployment with Functions enabled, and the embedded library refuses it.
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{/* end if */}
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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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`computed` and `transforms` 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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{/* if functions launched */}
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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. It needs a deployment with Functions
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enabled.
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{/* end if */}
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{/* if functions not launched */}
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When the value cannot be expressed in SQL -- an embedding, a model call, an
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image decode -- [Geneva](/geneva), the feature engineering package for LanceDB
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Enterprise, fills the column with a Python function instead.
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{/* end if */}
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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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{/* if functions launched */}
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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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{/* end if */}
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{/* if functions not launched */}
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<Card title="Geneva" icon="layer-group" href="/geneva">
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Fill a column with Python on LanceDB Enterprise.
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</Card>
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{/* end if */}
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</CardGroup>
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