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Jack Ye
734eb39dc3
Six groups were verbs -- Manage, Ingest, Train, Operate, Secure, Deploy -- and now name the thing rather than the action. Four more were sentence case. The page labels that were verb phrases follow: Add Computed Column becomes Computed Columns, Refresh Column becomes Column Refresh, Configuring Storage becomes Configuration, and Explain becomes Query Plans, which is also more accurate now that the page covers analyze as well. SQL statement pages take the keyword -- SELECT, DDL, DML, REFRESH, SHOW, EXPLAIN -- because a statement name is a noun and lowercase Select and Show read as instructions. Concept pages there keep noun phrases: Joins, Window Functions, Subqueries, Operators. Two duplicate labels are resolved on the way. Reranking sat in Semantic Search labelled Overview despite being the only page on its subject, and the Enterprise client page sat in SQL Reference labelled SQL next to the reference Overview; it is Client Connection now. Title case leaves prepositions and articles lowercase, so Needle in a Haystack and Hybrid Search on BEIR are already correct and unchanged.
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: "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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---
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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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- **`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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