$Revision: 3601 $
Copyright 2002-2010 Bob Preston and Fred Toussi. Permission is granted to distribute this document without any alteration under the terms of the HSQLDB license. Additional permission is granted to the HSQL Development Group to distribute this document with or without alterations under the terms of the HSQLDB license.
$Date: 2010-05-31 20:17:47 -0400 (Mon, 31 May 2010) $
Table of Contents
Text Table support for HSQLDB was originally developed by Bob Preston independently from the Project. Subsequently Bob joined the Project and incorporated this feature into version 1.7.0, with a number of enhancements, especially the use of conventional SQL commands for specifying the files used for Text Tables.
In a nutshell, Text Tables are CSV or other delimited files treated as SQL tables. Any ordinary CSV or other delimited file can be used. The full range of SQL queries can be performed on these files, including SELECT, INSERT, UPDATE and DELETE. Indexes and unique constraints can be set up, and foreign key constraints can be used to enforce referential integrity between Text Tables themselves or with conventional tables.
The delimited file can be created by the engine, or an existing file can be used.
HyperSQL with Text Table support is the only comprehensive solution that employs the power of SQL and the universal reach of JDBC to handle data stored in text files.
Text Tables are defined similarly to conventional tables with the added TEXT keyword:
CREATE TEXT TABLE <tablename> (<column definition> [<constraint definition>])
The table is at first empty and cannot be written to. An additional SET command specifies the file and the separator character that the Text table uses:
SET TABLE <tablename> SOURCE <quoted_filename_and_options> [DESC]
Text Tables cannot be created in mem: (all-in-memory) databases (databases that have no script file).
A Text table without a file assigned to it is READ ONLY and EMPTY.
Reassigning a Text Table definition to a new file has implications in the following areas:
The user is required to be an administrator.
Existing transactions are committed at this point.
Constraints, including foreign keys referencing this table, are kept intact. It is the responsibility of the administrator to ensure their integrity.
The new source file is scanned and indexes are built when it is assigned to the table. At this point any violation of NOT NULL, UNIQUE or PRIMARY KEY constraints are caught and the assignment is aborted. However, foreign key constraints are not checked at the time of assignment or reassignment of the source file.
Empty fields are treated as NULL. These are fields where there is nothing or just spaces between the separators.
Quoted empty strings are treated as empty strings.
The default field separator is a comma (,). A different field separator can be specified within the SET TABLE SOURCE statement. For example, to change the field separator for the table mytable to a vertical bar, place the following in the SET TABLE SOURCE statement, for example:
SET TABLE mytable SOURCE "myfile;fs=|"
Since HSQLDB treats CHAR and VARCHAR strings the same, the ability to assign a different separator to the latter is provided. When a different separator is assigned to a VARCHAR, it will terminate any CSV field of that type. For example, if the first field is CHAR, and the second field VARCHAR, and the separator fs has been defined as the pipe (|) and vs as the period (.) then the data in the CSV file for a row will look like:
First field data|Second field data.Third field data
This facility in effect offers an extra, special separator which can be used in addition to the global separator. The following example shows how to change the default separator to the pipe (|), VARCHAR separator to the period (.) within a SET TABLE SOURCE statement:
SET TABLE mytable SOURCE "myfile;fs=|;vs=."
HSQLDB also recognises the following special indicators for separators:
special indicators for separators
newline - Used as an end anchor (like $ in regular expressions)
a Unicode character specified in hexadecimal
Furthermore, HSQLDB provides csv file support with three
additional boolean options:
ignore_first option (default false) tells HSQLDB to
ignore the first line in a file. This option is used when the first line
of the file contains column headings. The
option (default false) tells the program that it should use quotes
around all character fields when writing to the source file. The
quoted option (default true) uses quotes only when
necessary to distinguish a field that contains the separator character.
It can be set to false to prevent the use of quoting altogether and
treat quote characters as normal characters. These options may be
specified within the
SET TABLE SOURCE
SET TABLE mytable SOURCE "myfile;ignore_first=true;all_quoted=true"
When the default options
quoted=true are in
force, fields that are written to a line of the csv file will be quoted
only if they contain the separator or the quote character. The quote
character is doubled when used inside a string. When
the quote character is not doubled. With this option, it is not possible
to insert any string containing the separator into the table, as it
would become impossible to distinguish from a separator. While reading
an existing data source file, the program treats each individual field
separately. It determines that a field is quoted only if the first
character is the quote character. It interprets the rest of the field on
The character encoding for the source file is
by default. To support UNICODE or source files prepared with
different encodings this can be changed to
any other encoding. The default is
encoding=UTF-8 or other supported
encodings can be used.
Finally, HSQLDB provides the ability to read a text file as READ ONLY, by placing the keyword "DESC" at the end of the SET TABLE SOURCE statement:
SET TABLE mytable SOURCE "myfile" DESC
Text table source files are cached in memory. The maximum number
of rows of data that are in memory at any time is controlled by the
textdb.cache_scale property. The default value for
textdb.cache_scale is 10 and can be changed by
setting the property in the .properties file for the database. The
number of rows in memory is calculated as 3*(2**scale), which translates
to 3072 rows for the default textdb.cache_scale setting (10). The
property can also be set for individual text tables:
SET TABLE mytable SOURCE "myfile;ignore_first=true;all_quoted=true;cache_scale=12"
Text tables may be disconnected from their underlying data source, i.e. the text file.
You can explicitly disconnect a text table from its file by issuing the following statement:
SET TABLE mytable SOURCE OFF
mytable will be empty and
read-only. However, the data source description will be preserved, and
the table can be re-connected to it with
SET TABLE mytable SOURCE ON
When a database is opened, if the source file for an existing text table is missing the table remains disconnected from its data source, but the source description is preserved. This allows the missing source file to be added to the directory and the table re-connected to it with the above command.
Disconnecting text tables from their source has several uses. While disconnected, the text source can be edited outside HSQLDB provided data integrity is respected. When large text sources are used, and several constraints or indexes need to be created on the table, it is possible to disconnect the source during the creation of constraints and indexes and reduce the time it takes to perform the operation.
The following information applies to the usage of text tables.
Text File Issues
File locations are restricted to below the directory that
contains the database, unless the
textdb.allow_full_path property is set true as a
Java system property. This feature is for security, otherwise an admin
database user may be able to open random files.
Blank lines are allowed anywhere in the text file, and are ignored.
It is possible to define a primary key, identity column, unique, foreign key and check constraints for text tables.
When a table source file is used with the
ignore_first=true option, the first, ignored line is
replaced with a blank line after a SHUTDOWN COMPACT, unless the SOURCE
HEADER statement has been used.
An existing table source file may include CHARACTER fields that do not begin with the quote character but contain instances of the quote character. These fields are read as literal strings. Alternatively, if any field begins with the quote character, then it is interpreted as a quoted string that should end with the quote character and any instances of the quote character within the string is doubled. When any field containing the quote character or the separator is written out to the source file by the program, the field is enclosed in quote character and any instance of the quote character inside the field is doubled.
Inserts or updates of CHARACTER type field values are allowed with strings that contains the linefeed or the carriage return character. This feature is disabled when both quoted and all_quoted properties are false.
ALTER TABLE commands that add or drop columns or constraints (apart from check constraints) are not supported with text tables that are connected to a source. First use the SET TABLE <name> SOURCE OFF, make the changes, then turn the source ON.
Complete list of supported global properties in *.properties files
Text tables fully support transactions. New or changed rows that have not been committed are not updated in the source file. Therefore the source file always contains committed rows.
However, text tables are not as resilient to machine crashes as other types of tables. If the crash happens while the text source is being written to, the text source may contain only some of the changes made during a committed transaction. With other types of tables, additional mechanisms ensure the integrity of the data and this situation will not arise.
$Revision: 3601 $