Documentation Source Text

<li><p>
Use [CREATE INDEX] and [CREATE TRIGGER] to reconstruct indexes and triggers
associated with table X.  Perhaps use the old format of the triggers and
indexes saved from step 1 above as a guide, making changes as appropriate
for the alteration.

<li><p>
If foreign key constraints were originally turned on using (prior to
step 4) then run [PRAGMA foreign_key_check] to verify that the schema
change did not break any foreign key contraints, and run
[PRAGMA foreign_keys | PRAGMA foreign_keys=ON] to reenable foreign key
constraints.

<li><p>If any views refer to table X in a way that is affected by the
schema change, then drop those views using [DROP VIEW] and recreate them
with whatever changes are necessary to accomodate the schema change
using [CREATE VIEW].

</ol>

<p>If desired, all of the steps above can be carried out inside of a
[BEGIN | transaction] so that the schema change is atomic.

<p>The procedure above is completely general and will work even if the
schema change causes the information stored in the table to change.
So the full procedure above is appropriate for dropping a column,
changing the order of columns, adding or removing a UNIQUE constraint
or PRIMARY KEY, adding CHECK or FOREIGN KEY or NOT NULL constraints,
or changing the datatype for a column, for example.  However, a simpler
and faster procedure can optionally be used for
some changes that do no affect the on-disk content in any way.
The following simpler procedure is appropriate for removing
CHECK or FOREIGN KEY or NOT NULL constraints,
renaming columns, or adding or removing or changing default values on
a column.

<ol>
<li><p> Run [PRAGMA schema_version] to determine the current schema
version number.  This number will be needed for step 5 below.

<li><p> Active schema editing using 
[PRAGMA writable_schema | PRAGMA writable_schema=ON].

<li><p> Run an [UPDATE] statement to change the definition of table X
in the [sqlite_master table]: 
UPDATE sqlite_master SET sql=... WHERE type='table' AND name='X';
<p><em>Caution:</em>  Making a change to the sqlite_master table like this will
render the database corrupt and unreadable if the change contains

</p>

<p>
The last two queries take the same amount of time, in our example.
So which index, Idx1 or Idx2, will SQLite choose?  If the
[ANALYZE] command has been run on the database, so that SQLite has
had an opportunity to gather statistics about the available indices,
then SQLite will know that the Idx1 table usually narrows the search
down to a single item (our example of fruit='Orange' is the exception
to this rule) where as the Idx table will normally only narrow the 
search down to two rows.  So, if all else is equal, SQLite will
choose Idx1 with the hope of narrowing the search to as small
a number of rows as possible.  This choice is only possible because
of the statistics provided by [ANALYZE].  If [ANALYZE] has not been
run then the choice of which index to use is arbitrary.
</p>