Virtuoso supports mapping files of comma separated values as tables from version 7.00.3206. This makes bulk load of relational data straightforward and very fast. This also makes it possible to query data in files without any loading into the database. The latter is specially convenient if data is queried just once, where loading and subsequently dropping the data would be needless overhead. Also arbitrary ETL transformations can be expressed in SQL, reading from a file table and inserting into a database table. This includes the whole range of SQL functionality, including intermediate aggregations and the like.
This feature deprecates the CSV load functions in previous versions, e.g. csv_load.
Queries can freely mix file tables and regular tables. Joins with file tables make hash join plans where the smaller file is copied into a memory resident hash table. For very large hash tables, partitioned hash join can be used, thus arbitrary joining between files is possible, fast and convenient.
A file table is declared with a regular create table statement. If the table definition includes a primary key, the file is assumed to be sorted as declared. In most cases a primary key should not be declared when creating a file table. After the create table statement, the procedure fs_set_file declares that the table is in fact a file. In a cluster setting the file should be visible to all the server processes via a shared file system.
For example, to bulk load the TPC-H part table:
CREATE TABLE PART_F ( P_PARTKEY INTEGER NOT NULL, P_NAME VARCHAR(55) NOT NULL, P_MFGR CHAR(25) NOT NULL, P_BRAND CHAR(10) NOT NULL, P_TYPE VARCHAR(25) NOT NULL, P_SIZE INTEGER NOT NULL, P_CONTAINER CHAR(10) NOT NULL, P_RETAILPRICE double precision NOT NULL, P_COMMENT VARCHAR(23) NOT NULL ); ft_set_file ('PART_F', 'src/part.tbl', delimiter => '|'); -- turn off logging and set insert mode to auto committing, non-transactional log_enable (2); -- read the file and insert into the previously create part table insert into part select * from part_f;
The ft_set_file procedure takes the table name as first argument and a file system path relative to the server's working directory as the second argument. Optional arguments allow specifying a delimiter, newline character and escape character.
create procedure ft_set_file (in tb varchar, in fname varchar, in delimiter varchar, in newline varchar := '\n', in esc varchar := null)
Creating a file table requires dba group privileges and the file is subject to the file system access limitations that apply to file_to_string and other file system access functions.
The newline and escape characters need to be single character strings. A newline or escape character following the escape character is added to the parsed input without its special interpretation.
Each column in the CSV file is expected to end with the delimiter character. A field of zero length is considered a SQL NULL value, i.e. if two delimiters are adjacent or if a line begins with the delimiter, the field is considered NULL. The text in the field is parsed according to the data type declared for the column whose position in the create table corresponds to the field position on the line. The parsing is as by the SQL cast function from a varchar value. If the cast fails the line is silently ignored.
When a table is declared as a file table, the file is sampled and statistics are written into the sys_col_stat table. In this way the system is capable of making correct query plans involving joins of file tables. For this reason, the file should exist and have the relevant content when the file table is created.
A file table will be read in parallel on multiple threads if a normal table would be read in parallel in the same situation. In a single server, the ThreadsPerQuery (enable_qp in sys_stat) setting controls the number of threads used. In a cluster setting each elastic slice corresponds to a fraction of the file. This is why the file is expected to be visible from all the servers.
If a table had previous content and subsequently was declared a file table, the previous content will no longer be visible. The file table may cease to be a file table by the dba deleting the corresponding row from sys_file_table and calling __ddl_changed on the file table. At this point the previous, databases resident content of the table will be visible again. However the statistics gathered from the file will still shadow the statistics of the table.
It is often the case that many CSV files have the same structure. For these cases it is sufficient to create a single table and attach it to one of the files with ft_set_file. This file will provide the statistics. In order to read a different file, one can use the FROM clause in the TABLE OPTION clause of in the FROM clause of a select statement, as follows:
-- get the count of well formed part rows from the part.tbl file SELECT COUNT (*) FROM part_f; -- do the same from src/part.tbl.2 SELECT COUNT (*) FROM part_f TABLE OPTION (FROM 'src/part.tbl.2');
The value of the FROM option can be an arbitrary expression, however it must be independent of values bound by the containing statement. It may depend on variables bound in an enclosing stored procedure or from parameters but it cannot join to a column of another table in the same select statement.
Arbitrary search conditions can be applied to file tables, for example:
SELECT p_name, p_size, p_partkey FROM part_f WHERE p_name LIKE '%green%';
Joins are possible, for example:
SELECT SUM (ps_availqty) FROM partsupp, part_f WHERE ps_partkey = p_partkey AND p_name LIKE '%green%';
This would join the partsupp table to the part file.
File tables are not cached by the DBMS, they are read from the OS every time they are needed. A single query will not read the same file multiple times, unless in the case of a partitioned (multtipass) hash join. Nested loop joins are not applied to file tables. File tables cannot be updated and no transaction control applies to them. Select permissions can be granted as for any other table.