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Fetch records from a previously executed query

Source: R/dbFetch.R, R/fetch.R
dbFetch.Rd

Fetch the next n elements (rows) from the result set and return them as a data.frame.

Methods in other packages

This documentation page describes the generics. Refer to the documentation pages linked below for the documentation for the methods that are implemented in various backend packages.

Usage

dbFetch(res, n = -1, ...)

fetch(res, n = -1, ...)

Arguments

res

An object inheriting from DBIResult, created by dbSendQuery().

n

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.

Value

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.

Details

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.

The data retrieval flow

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.

  1. Use dbSendQuery() to create a result set object of class DBIResult.

  2. 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.

  3. Optionally, use dbColumnInfo() to retrieve the structure of the result set without retrieving actual data.

  4. Use dbFetch() to get the entire result set, a page of results, or the remaining rows. Fetching zero rows is also possible to retrieve 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.

  5. Use dbHasCompleted() to tell when you're done. This method returns TRUE if no more rows are available for fetching.

  6. Repeat the last four steps as necessary.

  7. 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.

Failure modes

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 DBI::dbSendStatement() can be fetched and return an empty data frame, with a warning.

Specification

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:

  • lubridate::Date for dates (also applies to the return value of the SQL function current_date)

  • lubridate::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

See also

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()

Examples

con <- dbConnect(RSQLite::SQLite(), ":memory:")

dbWriteTable(con, "mtcars", mtcars)

# Fetch all results
rs <- dbSendQuery(con, "SELECT * FROM mtcars WHERE cyl = 4")
dbFetch(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)

# Fetch in chunks
rs <- dbSendQuery(con, "SELECT * FROM mtcars")
while (!dbHasCompleted(rs)) {
  chunk <- dbFetch(rs, 10)
  print(nrow(chunk))
}
#> [1] 10
#> [1] 10
#> [1] 10
#> [1] 2

dbClearResult(rs)
dbDisconnect(con)