mirror of
https://github.com/sqlite/sqlite.git
synced 2025-08-01 06:27:03 +03:00
Add new file ext/fts3/README.content, describing the experimental FTS4 content option.
FossilOrigin-Name: 13a9d085e1a5654a97b8d26bae7182ca6c0c237b
This commit is contained in:
dan
178
ext/fts3/README.content
Normal file
178
ext/fts3/README.content
Normal file
@ -0,0 +1,178 @@
|
||||
|
||||
FTS4 CONTENT OPTION
|
||||
|
||||
Normally, in order to create a full-text index on a dataset, the FTS4
|
||||
module stores a copy of all indexed documents in a specially created
|
||||
database table.
|
||||
|
||||
As of SQLite version 3.7.9, FTS4 supports a new option - "content" -
|
||||
designed to extend FTS4 to support the creation of full-text indexes where:
|
||||
|
||||
* The indexed documents are not stored within the SQLite database
|
||||
at all (a "contentless" FTS4 table), or
|
||||
|
||||
* The indexed documents are stored in a database table created and
|
||||
managed by the user (an "external content" FTS4 table).
|
||||
|
||||
Because the indexed documents themselves are usually much larger than
|
||||
the full-text index, the content option can sometimes be used to achieve
|
||||
significant space savings.
|
||||
|
||||
CONTENTLESS FTS4 TABLES
|
||||
|
||||
In order to create an FTS4 table that does not store a copy of the indexed
|
||||
documents at all, the content option should be set to an empty string.
|
||||
For example, the following SQL creates such an FTS4 table with three
|
||||
columns - "a", "b", and "c":
|
||||
|
||||
CREATE VIRTUAL TABLE t1 USING fts4(content="", a, b, c);
|
||||
|
||||
Data can be inserted into such an FTS4 table using an INSERT statements.
|
||||
However, unlike ordinary FTS4 tables, the user must supply an explicit
|
||||
integer docid value. For example:
|
||||
|
||||
-- This statement is Ok:
|
||||
INSERT INTO t1(docid, a, b, c) VALUES(1, 'a b c', 'd e f', 'g h i');
|
||||
|
||||
-- This statement causes an error, as no docid value has been provided:
|
||||
INSERT INTO t1(a, b, c) VALUES('j k l', 'm n o', 'p q r');
|
||||
|
||||
It is not possible to UPDATE or DELETE a row stored in a contentless FTS4
|
||||
table. Attempting to do so is an error.
|
||||
|
||||
Contentless FTS4 tables also support SELECT statements. However, it is
|
||||
an error to attempt to retrieve the value of any table column other than
|
||||
the docid column. The auxiliary function matchinfo() may be used, but
|
||||
snippet() and offsets() may not. For example:
|
||||
|
||||
-- The following statements are Ok:
|
||||
SELECT docid FROM t1 WHERE t1 MATCH 'xxx';
|
||||
SELECT docid FROM t1 WHERE a MATCH 'xxx';
|
||||
SELECT matchinfo(t1) FROM t1 WHERE t1 MATCH 'xxx';
|
||||
|
||||
-- The following statements all cause errors, as the value of columns
|
||||
-- other than docid are required to evaluate them.
|
||||
SELECT * FROM t1;
|
||||
SELECT a, b FROM t1 WHERE t1 MATCH 'xxx';
|
||||
SELECT docid FROM t1 WHERE a LIKE 'xxx%';
|
||||
SELECT snippet(t1) FROM t1 WHERE t1 MATCH 'xxx';
|
||||
|
||||
Errors related to attempting to retrieve column values other than docid
|
||||
are runtime errors that occur within sqlite3_step(). In some cases, for
|
||||
example if the MATCH expression in a SELECT query matches zero rows, there
|
||||
may be no error at all even if a statement does refer to column values
|
||||
other than docid.
|
||||
|
||||
EXTERNAL CONTENT FTS4 TABLES
|
||||
|
||||
An "external content" FTS4 table is similar to a contentless table, except
|
||||
that if evaluation of a query requires the value of a column other than
|
||||
docid, FTS4 attempts to retrieve that value from a table (or view, or
|
||||
virtual table) nominated by the user (hereafter referred to as the "content
|
||||
table"). The FTS4 module never writes to the content table, and writing
|
||||
to the content table does not affect the full-text index. It is the
|
||||
responsibility of the user to ensure that the content table and the
|
||||
full-text index are consistent.
|
||||
|
||||
An external content FTS4 table is created by setting the content option
|
||||
to the name of a table (or view, or virtual table) that may be queried by
|
||||
FTS4 to retrieve column values when required. If the nominated table does
|
||||
not exist, then an external content table behaves in the same way as
|
||||
a contentless table. For example:
|
||||
|
||||
CREATE TABLE t2(id INTEGER PRIMARY KEY, a, b, c);
|
||||
CREATE VIRTUAL TABLE t3 USING fts4(content="t2", a, c);
|
||||
|
||||
Assuming the nominated table does exist, then its columns must be the same
|
||||
as or a superset of those defined for the FTS table.
|
||||
|
||||
When a users query on the FTS table requires a column value other than
|
||||
docid, FTS attempts to read this value from the corresponding column of
|
||||
the row in the content table with a rowid value equal to the current FTS
|
||||
docid. Or, if such a row cannot be found in the content table, a NULL
|
||||
value is used instead. For example:
|
||||
|
||||
CREATE TABLE t2(id INTEGER PRIMARY KEY, a, b, c, d);
|
||||
CREATE VIRTUAL TABLE t3 USING fts4(content="t2", b, c);
|
||||
|
||||
INSERT INTO t2 VALUES(2, 'a b', 'c d', 'e f');
|
||||
INSERT INTO t2 VALUES(3, 'g h', 'i j', 'k l');
|
||||
INSERT INTO t3(docid, b, c) SELECT id, b, c FROM t2;
|
||||
|
||||
-- The following query returns a single row with two columns containing
|
||||
-- the text values "i j" and "k l".
|
||||
--
|
||||
-- The query uses the full-text index to discover that the MATCH
|
||||
-- term matches the row with docid=3. It then retrieves the values
|
||||
-- of columns b and c from the row with rowid=3 in the content table
|
||||
-- to return.
|
||||
--
|
||||
SELECT * FROM t3 WHERE t3 MATCH 'k';
|
||||
|
||||
-- Following the UPDATE, the query still returns a single row, this
|
||||
-- time containing the text values "xxx" and "yyy". This is because the
|
||||
-- full-text index still indicates that the row with docid=3 matches
|
||||
-- the FTS4 query 'k', even though the documents stored in the content
|
||||
-- table have been modified.
|
||||
--
|
||||
UPDATE t2 SET b = 'xxx', c = 'yyy' WHERE rowid = 3;
|
||||
SELECT * FROM t3 WHERE t3 MATCH 'k';
|
||||
|
||||
-- Following the DELETE below, the query returns one row containing two
|
||||
-- NULL values. NULL values are returned because FTS is unable to find
|
||||
-- a row with rowid=3 within the content table.
|
||||
--
|
||||
DELETE FROM t2;
|
||||
SELECT * FROM t3 WHERE t3 MATCH 'k';
|
||||
|
||||
When a row is deleted from an external content FTS4 table, FTS4 needs to
|
||||
retrieve the column values of the row being deleted from the content table.
|
||||
This is so that FTS4 can update the full-text index entries for each token
|
||||
that occurs within the deleted row to indicate that that row has been
|
||||
deleted. If the content table row cannot be found, or if it contains values
|
||||
inconsistent with the contents of the FTS index, the results can be difficult
|
||||
to predict. The FTS index may be left containing entries corresponding to the
|
||||
deleted row, which can lead to seemingly nonsensical results being returned
|
||||
by subsequent SELECT queries. The same applies when a row is updated, as
|
||||
internally an UPDATE is the same as a DELETE followed by an INSERT.
|
||||
|
||||
Instead of writing separately to the full-text index and the content table,
|
||||
some users may wish to use database triggers to keep the full-text index
|
||||
up to date with respect to the set of documents stored in the content table.
|
||||
For example, using the tables from earlier examples:
|
||||
|
||||
CREATE TRIGGER t2_bu BEFORE UPDATE ON t2 BEGIN
|
||||
DELETE FROM t3 WHERE docid=old.rowid;
|
||||
END;
|
||||
CREATE TRIGGER t2_bd BEFORE DELETE ON t2 BEGIN
|
||||
DELETE FROM t3 WHERE docid=old.rowid;
|
||||
END;
|
||||
|
||||
CREATE TRIGGER t2_bu AFTER UPDATE ON t2 BEGIN
|
||||
INSERT INTO t3(docid, b, c) VALUES(new.rowid, new.b, new.c);
|
||||
END;
|
||||
CREATE TRIGGER t2_bd AFTER INSERT ON t2 BEGIN
|
||||
INSERT INTO t3(docid, b, c) VALUES(new.rowid, new.b, new.c);
|
||||
END;
|
||||
|
||||
The DELETE trigger must be fired before the actual delete takes place
|
||||
on the content table. This is so that FTS4 can still retrieve the original
|
||||
values in order to update the full-text index. And the INSERT trigger must
|
||||
be fired after the new row is inserted, so as to handle the case where the
|
||||
rowid is assigned automatically within the system. The UPDATE trigger must
|
||||
be split into two parts, one fired before and one after the update of the
|
||||
content table, for the same reasons.
|
||||
|
||||
FTS4 features a special command similar to the 'optimize' command that
|
||||
deletes the entire full-text index and rebuilds it based on the current
|
||||
set of documents in the content table. Assuming again that "t3" is the
|
||||
name of the external content FTS4 table, the command is:
|
||||
|
||||
INSERT INTO t3(t3) VALUES('rebuild');
|
||||
|
||||
This command may also be used with ordinary FTS4 tables, although it may
|
||||
only be useful if the full-text index has somehow become corrupt. It is an
|
||||
error to attempt to rebuild the full-text index maintained by a contentless
|
||||
FTS4 table.
|
||||
|
||||
|
||||
Reference in New Issue
Block a user