[ARROW-18266] [R] Make it more obvious how to read in a Parquet file with a different schema to the inferred one

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Component/s: R
• External issue URL: https://github.com/apache/arrow/issues/33445
• Assignee: Unassigned
• Reporter: Nicola Crane
• Created: 07/Nov/22 14:31
• Updated: 11/Jan/23 11:59

Description

It's not all that clear from our docs that if we want to read in a Parquet file and change the schema, we need to call the cast() method on the Table, e.g.

# Write out data
data <- tibble::tibble(x = c(letters[1:5], NA), y = 1:6)
data_with_schema <- arrow_table(data, schema = schema(x = string(), y = int64()))
write_parquet(data_with_schema, "data_with_schema.parquet")

# Read in data while specifying a schema
data_in <- read_parquet("data_with_schema.parquet", as_data_frame = FALSE)	
data_in$cast(target_schema = schema(x = string(), y = int32()))

We should document this more clearly. Pehaps we could even update the code here to automatically do some of this if we pass in a schema to the ... argument of read_parquet and the returned data doesn't match the desired schema?