[sqlite] Remove obsolete reference version 3.8.7.4.

third_party/sqlite/sqlite-src-3080704/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.
-
-