Fetch records from a previously executed query as an Arrow object
Source:R/dbFetchArrow.R
dbFetchArrow.Rd
Fetch the result set and return it as an Arrow object.
Use dbFetchArrowChunk()
to fetch results in chunks.
Arguments
- res
An object inheriting from DBI::DBIResultArrow, created by
dbSendQueryArrow()
.- ...
Other arguments passed on to methods.
Value
dbFetchArrow()
always returns an object coercible to a data.frame with
as many rows as records were fetched and as many
columns as fields in the result set,
even if the result is a single value
or has one
or zero rows.
The data retrieval flow for Arrow streams
This section gives a complete overview over the flow for the execution of queries that return tabular data as an Arrow stream.
Most of this flow, except repeated calling of dbBindArrow()
or dbBind()
,
is implemented by dbGetQueryArrow()
,
which should be sufficient
unless you have a parameterized query that you want to reuse.
This flow requires an active connection established by dbConnect()
.
See also vignette("dbi-advanced")
for a walkthrough.
Use
dbSendQueryArrow()
to create a result set object of class DBIResultArrow.Optionally, bind query parameters with
dbBindArrow()
ordbBind()
. This is required only if the query contains placeholders such as?
or$1
, depending on the database backend.Use
dbFetchArrow()
to get a data stream.Repeat the last two steps as necessary.
Use
dbClearResult()
to clean up the result set object. This step is mandatory even if no rows have been fetched or if an error has occurred during the processing. It is good practice to useon.exit()
orwithr::defer()
to ensure that this step is always executed.
Specification
Fetching multi-row queries with one
or more columns by default returns the entire result.
The object returned by dbFetchArrow()
can also be passed to
nanoarrow::as_nanoarrow_array_stream()
to create a nanoarrow
array stream object that can be used to read the result set
in batches.
The chunk size is implementation-specific.
See also
Close the result set with dbClearResult()
as soon as you
finish retrieving the records you want.
Other DBIResultArrow generics:
DBIResultArrow-class
,
dbBind()
,
dbClearResult()
,
dbFetchArrowChunk()
,
dbHasCompleted()
,
dbIsValid()
Other data retrieval generics:
dbBind()
,
dbClearResult()
,
dbFetch()
,
dbFetchArrowChunk()
,
dbGetQuery()
,
dbGetQueryArrow()
,
dbHasCompleted()
,
dbSendQuery()
,
dbSendQueryArrow()
Examples
con <- dbConnect(RSQLite::SQLite(), ":memory:")
dbWriteTable(con, "mtcars", mtcars)
# Fetch all results
rs <- dbSendQueryArrow(con, "SELECT * FROM mtcars WHERE cyl = 4")
as.data.frame(dbFetchArrow(rs))
#> mpg cyl disp hp drat wt qsec vs am gear carb
#> 1 22.8 4 108.0 93 3.85 2.320 18.61 1 1 4 1
#> 2 24.4 4 146.7 62 3.69 3.190 20.00 1 0 4 2
#> 3 22.8 4 140.8 95 3.92 3.150 22.90 1 0 4 2
#> 4 32.4 4 78.7 66 4.08 2.200 19.47 1 1 4 1
#> 5 30.4 4 75.7 52 4.93 1.615 18.52 1 1 4 2
#> 6 33.9 4 71.1 65 4.22 1.835 19.90 1 1 4 1
#> 7 21.5 4 120.1 97 3.70 2.465 20.01 1 0 3 1
#> 8 27.3 4 79.0 66 4.08 1.935 18.90 1 1 4 1
#> 9 26.0 4 120.3 91 4.43 2.140 16.70 0 1 5 2
#> 10 30.4 4 95.1 113 3.77 1.513 16.90 1 1 5 2
#> 11 21.4 4 121.0 109 4.11 2.780 18.60 1 1 4 2
dbClearResult(rs)
dbDisconnect(con)