dbFetchArrowChunk | R Documentation |
Fetch the next chunk of the result set and return it as an Arrow object.
The chunk size is implementation-specific.
Use dbFetchArrow()
to fetch all results.
dbFetchArrowChunk(res, ...)
res |
An object inheriting from DBIResultArrow, created by
|
... |
Other arguments passed on to methods. |
dbFetchArrowChunk()
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.
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()
or dbBind()
.
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 use on.exit()
or withr::defer()
to ensure that this step is always executed.
An attempt to fetch from a closed result set raises an error.
Fetching multi-row queries with one
or more columns returns the next chunk.
The size of the chunk is implementation-specific.
The object returned by dbFetchArrowChunk()
can also be passed to
nanoarrow::as_nanoarrow_array()
to create a nanoarrow array object.
The chunk size is implementation-specific.
Close the result set with dbClearResult()
as soon as you
finish retrieving the records you want.
Other DBIResultArrow generics:
DBIResultArrow-class
,
dbBind()
,
dbClearResult()
,
dbFetchArrow()
,
dbHasCompleted()
,
dbIsValid()
Other data retrieval generics:
dbBind()
,
dbClearResult()
,
dbFetch()
,
dbFetchArrow()
,
dbGetQuery()
,
dbGetQueryArrow()
,
dbHasCompleted()
,
dbSendQuery()
,
dbSendQueryArrow()
con <- dbConnect(RSQLite::SQLite(), ":memory:")
dbWriteTable(con, "mtcars", mtcars)
# Fetch all results
rs <- dbSendQueryArrow(con, "SELECT * FROM mtcars WHERE cyl = 4")
dbHasCompleted(rs)
as.data.frame(dbFetchArrowChunk(rs))
dbHasCompleted(rs)
as.data.frame(dbFetchArrowChunk(rs))
dbClearResult(rs)
dbDisconnect(con)
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