Skip to contents

Execute a query on a given database connection for retrieval via Arrow

Source: R/24-dbSendQueryArrow.R
dbSendQueryArrow.Rd

[Experimental]

The dbSendQueryArrow() method only submits and synchronously executes the SQL query to the database engine. It does not extract any records — for that you need to use the dbFetchArrow() method, and then you must call dbClearResult() when you finish fetching the records you need. For interactive use, you should almost always prefer dbGetQueryArrow(). Use dbSendQuery() or dbGetQuery() instead to retrieve the results 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

dbSendQueryArrow(conn, statement, ...)

Arguments

conn

A DBIConnection object, as returned by dbConnect().

statement

a character string containing SQL.

...

Other parameters passed on to methods.

Value

dbSendQueryArrow() returns an S4 object that inherits from DBI::DBIResultArrow. The result set can be used with DBI::dbFetchArrow() to extract records. Once you have finished using a result, make sure to clear it with DBI::dbClearResult().

Details

This method is for SELECT queries only. Some backends may support data manipulation queries through this method for compatibility reasons. However, callers are strongly encouraged to use dbSendStatement() for data manipulation statements.

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.

  1. Use dbSendQueryArrow() to create a result set object of class DBIResultArrow.

  2. 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.

  3. Use dbFetchArrow() to get a data stream.

  4. Repeat the last two steps as necessary.

  5. 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 error is raised when issuing a query over a closed or invalid connection, or if the query is not a non-NA string. An error is also raised if the syntax of the query is invalid and all query parameters are given (by passing the params argument) or the immediate argument is set to TRUE.

Additional arguments

The following arguments are not part of the dbSendQueryArrow() generic (to improve compatibility across backends) but are part of the DBI specification:

  • params (default: NULL)

  • immediate (default: NULL)

They must be provided as named arguments. See the "Specification" sections for details on their usage.

Specification

No warnings occur under normal conditions. When done, the DBIResult object must be cleared with a call to DBI::dbClearResult(). Failure to clear the result set leads to a warning when the connection is closed.

If the backend supports only one open result set per connection, issuing a second query invalidates an already open result set and raises a warning. The newly opened result set is valid and must be cleared with dbClearResult().

The param argument allows passing query parameters, see DBI::dbBind() for details.

Specification for the immediate argument

The immediate argument supports distinguishing between "direct" and "prepared" APIs offered by many database drivers. Passing immediate = TRUE leads to immediate execution of the query or statement, via the "direct" API (if supported by the driver). The default NULL means that the backend should choose whatever API makes the most sense for the database, and (if relevant) tries the other API if the first attempt fails. A successful second attempt should result in a message that suggests passing the correct immediate argument. Examples for possible behaviors:

  1. DBI backend defaults to immediate = TRUE internally

    1. A query without parameters is passed: query is executed

    2. A query with parameters is passed:

      1. params not given: rejected immediately by the database because of a syntax error in the query, the backend tries immediate = FALSE (and gives a message)

      2. params given: query is executed using immediate = FALSE

  2. DBI backend defaults to immediate = FALSE internally

    1. A query without parameters is passed:

      1. simple query: query is executed

      2. "special" query (such as setting a config options): fails, the backend tries immediate = TRUE (and gives a message)

    2. A query with parameters is passed:

      1. params not given: waiting for parameters via DBI::dbBind()

      2. params given: query is executed

See also

For updates: dbSendStatement() and dbExecute().

Other DBIConnection generics: DBIConnection-class, dbAppendTable(), dbAppendTableArrow(), dbCreateTable(), dbCreateTableArrow(), dbDataType(), dbDisconnect(), dbExecute(), dbExistsTable(), dbGetException(), dbGetInfo(), dbGetQuery(), dbGetQueryArrow(), dbIsReadOnly(), dbIsValid(), dbListFields(), dbListObjects(), dbListResults(), dbListTables(), dbQuoteIdentifier(), dbReadTable(), dbReadTableArrow(), dbRemoveTable(), dbSendQuery(), dbSendStatement(), dbUnquoteIdentifier(), dbWriteTable(), dbWriteTableArrow()

Other data retrieval generics: dbBind(), dbClearResult(), dbFetch(), dbFetchArrow(), dbFetchArrowChunk(), dbGetQuery(), dbGetQueryArrow(), dbHasCompleted(), dbSendQuery()

Examples

# Retrieve data as arrow table
con <- dbConnect(RSQLite::SQLite(), ":memory:")

dbWriteTable(con, "mtcars", mtcars)
rs <- dbSendQueryArrow(con, "SELECT * FROM mtcars WHERE cyl = 4")
dbFetchArrow(rs)
#> <nanoarrow_array_stream struct<mpg: double, cyl: double, disp: double, hp: double, drat: double, wt: double, qsec: double, vs: double, am: double, gear: double, carb: double>>
#>  $ get_schema:function ()  
#>  $ get_next  :function (schema = x$get_schema(), validate = TRUE)  
#>  $ release   :function ()  
dbClearResult(rs)

dbDisconnect(con)