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Create a reader backed by R callbacks

Source: R/reader.R
custom_reader.Rd

Construct a reader whose behavior is defined entirely by R functions you supply. This makes it possible to plug in data sources that aren't provided natively by ggsql (e.g. an in-memory store, a custom HTTP API, a DBI connection, etc.) without touching the Rust side.

Usage

custom_reader(execute_sql, register = NULL, unregister = NULL)

Arguments

execute_sql

A function function(sql) that executes sql and returns either a data frame or a raw vector containing Arrow IPC stream bytes (as produced by nanoarrow::as_nanoarrow_array_stream() / arrow IPC writers).

register

Optional function(name, df, replace) that registers df as a table named name. replace is TRUE if the caller expects an existing table with the same name to be replaced.

unregister

Optional function(name) that removes a previously registered table.

Value

A Reader object, usable anywhere the other *_reader() constructors are accepted.

Details

Only execute_sql is required. If register or unregister are omitted, calling ggsql_register() / ggsql_unregister() on the returned reader raises an error.

See also

Other readers: duckdb_reader(), odbc_reader(), snowflake_reader()

Examples

# A trivial reader backed by a list of data frames in an environment,
# delegating the actual SQL engine to an in-memory DuckDB.
store <- new.env(parent = emptyenv())
backend <- duckdb_reader()
reader <- custom_reader(
  execute_sql = function(sql) ggsql_execute_sql(backend, sql),
  register = function(name, df, replace) {
    store[[name]] <- df
    ggsql_register(backend, df, name, replace = replace)
  },
  unregister = function(name) {
    rm(list = name, envir = store)
    ggsql_unregister(backend, name)
  }
)
ggsql_register(reader, mtcars, "cars")
ggsql_execute_sql(reader, "SELECT mpg, disp FROM cars LIMIT 3")
#>    mpg disp
#> 1 21.0  160
#> 2 21.0  160
#> 3 22.8  108