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, or to execute programs on the server
and read their output. The data file or program output must be in a format
that can be read by COPY FROM;
see COPY for details.
Access to 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. Relative paths are relative to the
data directory.
Either filename or program must be
specified, but not both.
program
Specifies the command to be executed. The standard output of this
command will be read as though COPY FROM PROGRAM were used.
Either program or filename must be
specified, but not both.
format
Specifies the data format,
the same as COPY's FORMAT option.
header
Specifies whether the data has a header line,
the same as COPY's HEADER option.
delimiter
Specifies the data delimiter character,
the same as COPY's DELIMITER option.
quote
Specifies the data quote character,
the same as COPY's QUOTE option.
escape
Specifies the data escape character,
the same as COPY's ESCAPE option.
null
Specifies the data null string,
the same as COPY's NULL option.
encoding
Specifies the data encoding,
the same as COPY's ENCODING option.
Note that while COPY allows options such as HEADER
to be specified without a corresponding value, the foreign table option
syntax requires a value to be present in all cases. To activate
COPY options typically written without a value, you can pass
the value TRUE, since all such options are Booleans.
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
table-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 FORCE_QUOTE option is
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 being a superuser or having the privileges
of the default role pg_read_server_files (to use a filename) or
the default role pg_execute_server_program (to use a program),
for security reasons: only certain users should be able to control which file is
read or which program is run. In principle regular users could be allowed to
change the other options, but that's not supported at present.
When specifying the program option, keep in mind that the option
string is executed by the shell. If you need to pass any arguments to the
command that come from an untrusted source, you must be careful to strip or
escape any characters that might have special meaning to the shell.
For security reasons, it is best to use a fixed command string, or at least
avoid passing any user input in it.
For a foreign table using file_fdw, EXPLAIN shows
the name of the file to be read or program to be run.
For a file, unless COSTS OFF is
specified, the file size (in bytes) is shown as well.
Example F.1. Create a Foreign Table for LightDB CSV Logs
One of the obvious uses for file_fdw is to make
the LightDB 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 ltlog.csv. First, install file_fdw
as an extension:
CREATE EXTENSION file_fdw;
Then create a foreign server:
CREATE SERVER ltlog 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 ltlog ( 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, backend_type text ) SERVER ltlog OPTIONS ( filename 'log/ltlog.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.