The file_fdw
module provides the foreign-data wrapper
file_fdw
, which can be used to access data
files in the server's file system. Data files must be in a format
that can be read by COPY FROM;
see COPY(7) for details.
Access to such data files is currently read-only.
A foreign table created using this wrapper can have the following options:
filename
Specifies the file to be read. Required. Must be an absolute path name.
format
Specifies the file's format,
the same as COPY's FORMAT
option.
header
Specifies whether the file has a header line,
the same as COPY's HEADER
option.
delimiter
Specifies the file's delimiter character,
the same as COPY's DELIMITER
option.
quote
Specifies the file's quote character,
the same as COPY's QUOTE
option.
escape
Specifies the file's escape character,
the same as COPY's ESCAPE
option.
null
Specifies the file's null string,
the same as COPY's NULL
option.
encoding
Specifies the file's encoding,
the same as COPY's ENCODING
option.
Note that while COPY allows options such as OIDS and HEADER to be specified without a corresponding value, the foreign data wrapper syntax requires a value to be present in all cases. To activate COPY options normally supplied without a value, you can instead pass the value TRUE.
A column of a foreign table created using this wrapper can have the following options:
force_not_null
This is a Boolean option. If true, it specifies that values of the
column should not be matched against the null string (that is, the
file-level null
option). This has the same effect
as listing the column in COPY's
FORCE_NOT_NULL
option.
force_null
This is a Boolean option. If true, it specifies that values of the
column which match the null string are returned as NULL
even if the value is quoted. Without this option, only unquoted
values matching the null string are returned as NULL
.
This has the same effect as listing the column in
COPY's FORCE_NULL
option.
COPY's OIDS
and
FORCE_QUOTE
options are currently not supported by
file_fdw
.
These options can only be specified for a foreign table or its columns, not
in the options of the file_fdw
foreign-data wrapper, nor in the
options of a server or user mapping using the wrapper.
Changing table-level options requires superuser privileges, for security reasons: only a superuser should be able to determine which file is read. In principle non-superusers could be allowed to change the other options, but that's not supported at present.
For a foreign table using file_fdw
, EXPLAIN shows
the name of the file to be read. Unless COSTS OFF
is
specified, the file size (in bytes) is shown as well.
Example F.1. Create a Foreign Table for PostgreSQL CSV Logs
One of the obvious uses for the file_fdw
is to make
the PostgreSQL activity log available as a table for querying. To
do this, first you must be logging to a CSV file, which here we
will call pglog.csv
. First, install file_fdw
as an extension:
CREATE EXTENSION file_fdw;
Then create a foreign server:
CREATE SERVER pglog FOREIGN DATA WRAPPER file_fdw;
Now you are ready to create the foreign data table. Using the CREATE FOREIGN TABLE command, you will need to define the columns for the table, the CSV file name, and its format:
CREATE FOREIGN TABLE pglog ( log_time timestamp(3) with time zone, user_name text, database_name text, process_id integer, connection_from text, session_id text, session_line_num bigint, command_tag text, session_start_time timestamp with time zone, virtual_transaction_id text, transaction_id bigint, error_severity text, sql_state_code text, message text, detail text, hint text, internal_query text, internal_query_pos integer, context text, query text, query_pos integer, location text, application_name text ) SERVER pglog OPTIONS ( filename '/home/josh/9.1/data/pg_log/pglog.csv', format 'csv' );
That's it — now you can query your log directly. In production, of course, you would need to define some way to deal with log rotation.