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third_party/sqlite/src/ext/fts3/README.content

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+
+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.
+
+