Fetch the next n elements (rows) from the result set and return them
as a data.frame.
DBI:::methods_as_rd("dbFetch")
dbFetch(res, n = -1, ...)fetch(res, n = -1, ...)
dbFetch() always returns 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.
Passing n = NA is supported and returns an arbitrary number of rows (at least one)
as specified by the driver, but at most the remaining rows in the result set.
An object inheriting from DBIResult, created by
dbSendQuery().
maximum number of records to retrieve per fetch. Use n = -1
or n = Inf
to retrieve all pending records. Some implementations may recognize other
special values.
Other arguments passed on to methods.
This section gives a complete overview over the flow for the execution of queries that return tabular data as data frames.
Most of this flow, except repeated calling of dbBind() or dbBindArrow(),
is implemented by dbGetQuery(), which should be sufficient
unless you want to access the results in a paged way
or 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 dbSendQuery() to create a result set object of class
DBIResult.
Optionally, bind query parameters with dbBind() or dbBindArrow().
This is required only if the query contains placeholders
such as ? or $1, depending on the database backend.
Optionally, use dbColumnInfo() to retrieve the structure of the result set
without retrieving actual data.
Use dbFetch() to get the entire result set, a page of results,
or the remaining rows.
Fetching zero rows is also possible to retrieeve the structure of the result set
as a data frame.
This step can be called multiple times.
Only forward paging is supported, you need to cache previous pages
if you need to navigate backwards.
Use dbHasCompleted() to tell when you're done.
This method returns TRUE if no more rows are available for fetching.
Repeat the last four 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.
If the n argument is not an atomic whole number
greater or equal to -1 or Inf, an error is raised,
but a subsequent call to dbFetch() with proper n argument succeeds.
Calling dbFetch() on a result set from a data manipulation query
created by dbSendStatement() can
be fetched and return an empty data frame, with a warning.
Fetching multi-row queries with one
or more columns by default returns the entire result.
Multi-row queries can also be fetched progressively
by passing a whole number (integer or
numeric)
as the n argument.
A value of Inf for the n argument is supported
and also returns the full result.
If more rows than available are fetched, the result is returned in full
without warning.
If fewer rows than requested are returned, further fetches will
return a data frame with zero rows.
If zero rows are fetched, the columns of the data frame are still fully
typed.
Fetching fewer rows than available is permitted,
no warning is issued when clearing the result set.
A column named row_names is treated like any other column.
The column types of the returned data frame depend on the data returned:
integer (or coercible to an integer) for integer values between -2^31 and 2^31 - 1,
with NA for SQL NULL values
numeric for numbers with a fractional component,
with NA for SQL NULL values
logical for Boolean values (some backends may return an integer);
with NA for SQL NULL values
character for text,
with NA for SQL NULL values
lists of raw for blobs with NULL entries for SQL NULL values
coercible using as.Date() for dates,
with NA for SQL NULL values
(also applies to the return value of the SQL function current_date)
coercible using hms::as_hms() for times,
with NA for SQL NULL values
(also applies to the return value of the SQL function current_time)
coercible using as.POSIXct() for timestamps,
with NA for SQL NULL values
(also applies to the return value of the SQL function current_timestamp)
If dates and timestamps are supported by the backend, the following R types are used:
Date for dates
(also applies to the return value of the SQL function current_date)
POSIXct for timestamps
(also applies to the return value of the SQL function current_timestamp)
R has no built-in type with lossless support for the full range of 64-bit or larger integers. If 64-bit integers are returned from a query, the following rules apply:
Values are returned in a container with support for the full range of
valid 64-bit values (such as the integer64 class of the bit64
package)
Coercion to numeric always returns a number that is as close as possible to the true value
Loss of precision when converting to numeric gives a warning
Conversion to character always returns a lossless decimal representation of the data
fetch() is provided for compatibility with older DBI clients - for all
new code you are strongly encouraged to use dbFetch(). The default
implementation for dbFetch() calls fetch() so that it is compatible with
existing code. Modern backends should implement for dbFetch() only.
Close the result set with dbClearResult() as soon as you
finish retrieving the records you want.
Other DBIResult generics:
DBIResult-class,
dbBind(),
dbClearResult(),
dbColumnInfo(),
dbGetInfo(),
dbGetRowCount(),
dbGetRowsAffected(),
dbGetStatement(),
dbHasCompleted(),
dbIsReadOnly(),
dbIsValid(),
dbQuoteLiteral(),
dbQuoteString()
Other data retrieval generics:
dbBind(),
dbClearResult(),
dbFetchArrow(),
dbFetchArrowChunk(),
dbGetQuery(),
dbGetQueryArrow(),
dbHasCompleted(),
dbSendQuery(),
dbSendQueryArrow()
if (FALSE) { # requireNamespace("RSQLite", quietly = TRUE)
con <- dbConnect(RSQLite::SQLite(), ":memory:")
dbWriteTable(con, "mtcars", mtcars)
# Fetch all results
rs <- dbSendQuery(con, "SELECT * FROM mtcars WHERE cyl = 4")
dbFetch(rs)
dbClearResult(rs)
# Fetch in chunks
rs <- dbSendQuery(con, "SELECT * FROM mtcars")
while (!dbHasCompleted(rs)) {
chunk <- dbFetch(rs, 10)
print(nrow(chunk))
}
dbClearResult(rs)
dbDisconnect(con)
}
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