Schema {arrow}R Documentation

Schema class

Description

A Schema is a list of Fields, which map names to Arrow data types. Create a Schema when you want to convert an R data.frame to Arrow but don't want to rely on the default mapping of R types to Arrow types, such as when you want to choose a specific numeric precision, or when creating a Dataset and you want to ensure a specific schema rather than inferring it from the various files.

Many Arrow objects, including Table and Dataset, have a $schema method (active binding) that lets you access their schema.

Usage

schema(...)

Arguments

...

named list of data types

Methods

Active bindings

R Metadata

When converting a data.frame to an Arrow Table or RecordBatch, attributes from the data.frame are saved alongside tables so that the object can be reconstructed faithfully in R (e.g. with as.data.frame()). This metadata can be both at the top-level of the data.frame (e.g. attributes(df)) or at the column (e.g. attributes(df$col_a)) or for list columns only: element level (e.g. attributes(df[1, "col_a"])). For example, this allows for storing haven columns in a table and being able to faithfully re-create them when pulled back into R. This metadata is separate from the schema (column names and types) which is compatible with other Arrow clients. The R metadata is only read by R and is ignored by other clients (e.g. Pandas has its own custom metadata). This metadata is stored in $metadata$r.

Since Schema metadata keys and values must be strings, this metadata is saved by serializing R's attribute list structure to a string. If the serialized metadata exceeds 100Kb in size, by default it is compressed starting in version 3.0.0. To disable this compression (e.g. for tables that are compatible with Arrow versions before 3.0.0 and include large amounts of metadata), set the option arrow.compress_metadata to FALSE. Files with compressed metadata are readable by older versions of arrow, but the metadata is dropped.

Examples


df <- data.frame(col1 = 2:4, col2 = c(0.1, 0.3, 0.5))
tab1 <- Table$create(df)
tab1$schema
tab2 <- Table$create(df, schema = schema(col1 = int8(), col2 = float32()))
tab2$schema


[Package arrow version 4.0.1 Index]