{"u":"lang_corefunc.html#round","x":"round(X)","s":0},
{"u":"lang_corefunc.html#round","x":"round(X,Y)","s":0},
{"u":"lang_corefunc.html#rtrim","x":"rtrim(X)","s":0},
{"u":"lang_corefunc.html#rtrim","x":"rtrim(X,Y)","s":0},
{"u":"lang_corefunc.html#soundex","x":"soundex(X)","s":0},
{"u":"lang_corefunc.html#sqlite_compileoption_get","x":"sqlite_compileoption_get(N)","s":0},
{"u":"lang_corefunc.html#sqlite_compileoption_used","x":"sqlite_compileoption_used(X)","s":0},

{"u":"lang_corefunc.html#sqlite_source_id","x":"sqlite_source_id()","s":0},
{"u":"lang_corefunc.html#sqlite_version","x":"sqlite_version()","s":0},
{"u":"lang_corefunc.html#substr","x":"substr(X,Y)","s":0},
{"u":"lang_corefunc.html#substr","x":"substr(X,Y,Z)","s":0},
{"u":"lang_corefunc.html#total_changes","x":"total_changes()","s":0},
{"u":"lang_corefunc.html#trim","x":"trim(X)","s":0},
{"u":"lang_corefunc.html#trim","x":"trim(X,Y)","s":0},
................................................................................
  The sqlite_compileoption_used() SQL function is a wrapper around the
  <a href="c3ref/compileoption_get.html">sqlite3_compileoption_used()</a> C/C++ function.
  When the argument X to sqlite_compileoption_used(X) is a string which
  is the name of a compile-time option, this routine returns true (1) or
  false (0) depending on whether or not that option was used during the
  build.
</dd>















<a name="sqlite_source_id"></a>
<dt><p><b>sqlite_source_id()</b></dt><dd><p>
  The sqlite_source_id() function returns a string that identifies the
  specific version of the source code that was used to build the SQLite
  library.  The string returned by sqlite_source_id() is
  the date and time that the source code was checked in followed by
  the SHA1 hash for that check-in.  This function is

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{"u":"lang_corefunc.html#round","x":"round(X,Y)","s":0},
{"u":"lang_corefunc.html#rtrim","x":"rtrim(X)","s":0},
{"u":"lang_corefunc.html#rtrim","x":"rtrim(X,Y)","s":0},
{"u":"lang_corefunc.html#soundex","x":"soundex(X)","s":0},
{"u":"lang_corefunc.html#sqlite_compileoption_get","x":"sqlite_compileoption_get(N)","s":0},
{"u":"lang_corefunc.html#sqlite_compileoption_used","x":"sqlite_compileoption_used(X)","s":0},
{"u":"lang_corefunc.html#sqlite_offset","x":"sqlite_offset(X)","s":0},
{"u":"lang_corefunc.html#sqlite_source_id","x":"sqlite_source_id()","s":0},
{"u":"lang_corefunc.html#sqlite_version","x":"sqlite_version()","s":0},
{"u":"lang_corefunc.html#substr","x":"substr(X,Y)","s":0},
{"u":"lang_corefunc.html#substr","x":"substr(X,Y,Z)","s":0},
{"u":"lang_corefunc.html#total_changes","x":"total_changes()","s":0},
{"u":"lang_corefunc.html#trim","x":"trim(X)","s":0},
{"u":"lang_corefunc.html#trim","x":"trim(X,Y)","s":0},
................................................................................
  The sqlite_compileoption_used() SQL function is a wrapper around the
  <a href="c3ref/compileoption_get.html">sqlite3_compileoption_used()</a> C/C++ function.
  When the argument X to sqlite_compileoption_used(X) is a string which
  is the name of a compile-time option, this routine returns true (1) or
  false (0) depending on whether or not that option was used during the
  build.
</dd>
<a name="sqlite_offset"></a>
<dt><p><b>sqlite_offset(<i>X</i>)</b></dt><dd><p>
  The sqlite_offset(X) function returns the byte offset in the database
  file for the beginning of the record from which value would be read.
  If X is not a column in an ordinary table, then sqlite_offset(X) returns
  NULL.  The value returned by sqlite_offset(X) might reference either the
  original table or an index, depending on the query.  If the value X would
  normally be extracted from an index, the sqlite_offset(X) returns the
  offset to the corresponding index record.  If the value X would be
  extracted from the original table, then sqlite_offset(X) returns the offset
  to the table record.

  <p>The sqlite_offset(X) SQL function is only available if SQLite is built
  using the <a href="compile.html#enable_offset_sql_func">-DSQLITE_ENABLE_OFFSET_SQL_FUNC</a> compile-time option.
</dd>
<a name="sqlite_source_id"></a>
<dt><p><b>sqlite_source_id()</b></dt><dd><p>
  The sqlite_source_id() function returns a string that identifies the
  specific version of the source code that was used to build the SQLite
  library.  The string returned by sqlite_source_id() is
  the date and time that the source code was checked in followed by
  the SHA1 hash for that check-in.  This function is

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{"u":"pragma.html#pragma_foreign_keys","x":"foreign_keys","s":0},
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{"u":"pragma.html#pragma_fullfsync","x":"fullfsync","s":0},

{"u":"pragma.html#pragma_ignore_check_constraints","x":"ignore_check_constraints","s":0},
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{"u":"pragma.html#pragma_page_count","x":"page_count","s":0},
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{"u":"pragma.html#pragma_parser_trace","x":"parser_trace","s":4},

{"u":"pragma.html#pragma_query_only","x":"query_only","s":0},
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{"u":"pragma.html#pragma_recursive_triggers","x":"recursive_triggers","s":0},
{"u":"pragma.html#pragma_reverse_unordered_selects","x":"reverse_unordered_selects","s":0},
{"u":"pragma.html#pragma_schema_version","x":"schema_version","s":5},
{"u":"pragma.html#pragma_secure_delete","x":"secure_delete","s":0},
................................................................................
</script>

<p>Notes:
<ol>
<li>Pragmas whose names are <s>struck through</s>
are deprecated. Do not use them. They exist
for historical compatibility.
<li>These pragmas are used for debugging SQLite and
are only available when SQLite is compiled using <a href="compile.html#debug">SQLITE_DEBUG</a>.
<li>These pragmas are used for testing SQLite and are not recommended
for use in application programs.</ol></p>
<a name="pragma_application_id"></a>
<h _id=pragma_application_id style="display:none"> PRAGMA application_id</h><hr>
    <p><b>PRAGMA </b><i>schema.</i><b>application_id;
     <br>PRAGMA </b><i>schema.</i><b>application_id = </b><i>integer </i><b>;</b>
  
................................................................................
       <br>PRAGMA fullfsync = </b><i>boolean</i><b>;</b></p>
    <p>Query or change the fullfsync flag. This flag
    determines whether or not the F_FULLFSYNC syncing method is used
    on systems that support it.  The default value of the fullfsync flag
    is off.  Only Mac OS X supports F_FULLFSYNC.</p>

    <p>See also <a href="pragma.html#pragma_checkpoint_fullfsync">checkpoint_fullfsync</a>.</p>










<a name="pragma_ignore_check_constraints"></a>
<h _id=pragma_ignore_check_constraints style="display:none"> PRAGMA ignore_check_constraints</h><hr>
    <p><b>PRAGMA ignore_check_constraints  = </b><i>boolean</i><b>;</b></p>

    <p>This pragma enables or disables the enforcement of CHECK constraints.
    The default setting is off, meaning that CHECK constraints are
    enforced by default.</p>
................................................................................

    <p>The size of the memory-mapped I/O region cannot be changed while
       the memory-mapped I/O region is in active use, to avoid unmapping
       memory out from under running SQL statements.  For this reason,
       the mmap_size pragma may be a no-op if the prior mmap_size is non-zero
       and there are other SQL statements running concurrently on the same
       <a href="c3ref/sqlite3.html">database connection</a>.</p>










<a name="pragma_optimize"></a>
<h _id=pragma_optimize style="display:none"> PRAGMA optimize</h><hr>
  <p><b>PRAGMA optimize;
       <br>PRAGMA optimize(</b><i>MASK</i><b>);
       <br>PRAGMA </b><i>schema</i><b>.optimize;
       <br>PRAGMA </b><i>schema</i><b>.optimize(</b><i>MASK</i><b>);</b></p>
  <p>Attempt to optimize the database.  All schemas are optimized in the 
................................................................................

    
    <p style='background-color: #f0e0ff;'>
    This pragma is intended for use when debugging SQLite itself.  It
    is only available when the <a href="compile.html#debug">SQLITE_DEBUG</a> compile-time option
    is used.</p>
  










<a name="pragma_query_only"></a>
<h _id=pragma_query_only style="display:none"> PRAGMA query_only</h><hr>
    <p><b>PRAGMA query_only;
      <br>PRAGMA query_only = </b><i>boolean</i><b>;</b></p>

    <p>The query_only pragma prevents all changes to database files when
    enabled.</p>

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{"u":"pragma.html#pragma_foreign_key_list","x":"foreign_key_list","s":0},
{"u":"pragma.html#pragma_foreign_keys","x":"foreign_keys","s":0},
{"u":"pragma.html#pragma_freelist_count","x":"freelist_count","s":0},
{"u":"pragma.html#pragma_full_column_names","x":"full_column_names","s":3},
{"u":"pragma.html#pragma_fullfsync","x":"fullfsync","s":0},
{"u":"pragma.html#pragma_function_list","x":"function_list","s":4},
{"u":"pragma.html#pragma_ignore_check_constraints","x":"ignore_check_constraints","s":0},
{"u":"pragma.html#pragma_incremental_vacuum","x":"incremental_vacuum","s":0},
{"u":"pragma.html#pragma_index_info","x":"index_info","s":0},
{"u":"pragma.html#pragma_index_list","x":"index_list","s":0},
{"u":"pragma.html#pragma_index_xinfo","x":"index_xinfo","s":0},
{"u":"pragma.html#pragma_integrity_check","x":"integrity_check","s":0},
{"u":"pragma.html#pragma_journal_mode","x":"journal_mode","s":0},
{"u":"pragma.html#pragma_journal_size_limit","x":"journal_size_limit","s":0},
{"u":"pragma.html#pragma_legacy_file_format","x":"legacy_file_format","s":0},
{"u":"pragma.html#pragma_locking_mode","x":"locking_mode","s":0},
{"u":"pragma.html#pragma_max_page_count","x":"max_page_count","s":0},
{"u":"pragma.html#pragma_mmap_size","x":"mmap_size","s":0},
{"u":"pragma.html#pragma_module_list","x":"module_list","s":4},
{"u":"pragma.html#pragma_optimize","x":"optimize","s":0},
{"u":"pragma.html#pragma_page_count","x":"page_count","s":0},
{"u":"pragma.html#pragma_page_size","x":"page_size","s":0},
{"u":"pragma.html#pragma_parser_trace","x":"parser_trace","s":4},
{"u":"pragma.html#pragma_pragma_list","x":"pragma_list","s":4},
{"u":"pragma.html#pragma_query_only","x":"query_only","s":0},
{"u":"pragma.html#pragma_quick_check","x":"quick_check","s":0},
{"u":"pragma.html#pragma_read_uncommitted","x":"read_uncommitted","s":0},
{"u":"pragma.html#pragma_recursive_triggers","x":"recursive_triggers","s":0},
{"u":"pragma.html#pragma_reverse_unordered_selects","x":"reverse_unordered_selects","s":0},
{"u":"pragma.html#pragma_schema_version","x":"schema_version","s":5},
{"u":"pragma.html#pragma_secure_delete","x":"secure_delete","s":0},
................................................................................
</script>

<p>Notes:
<ol>
<li>Pragmas whose names are <s>struck through</s>
are deprecated. Do not use them. They exist
for historical compatibility.
<li>These pragmas are only available in builds using non-standard
compile-time options.
<li>These pragmas are used for testing SQLite and are not recommended
for use in application programs.</ol></p>
<a name="pragma_application_id"></a>
<h _id=pragma_application_id style="display:none"> PRAGMA application_id</h><hr>
    <p><b>PRAGMA </b><i>schema.</i><b>application_id;
     <br>PRAGMA </b><i>schema.</i><b>application_id = </b><i>integer </i><b>;</b>
  
................................................................................
       <br>PRAGMA fullfsync = </b><i>boolean</i><b>;</b></p>
    <p>Query or change the fullfsync flag. This flag
    determines whether or not the F_FULLFSYNC syncing method is used
    on systems that support it.  The default value of the fullfsync flag
    is off.  Only Mac OS X supports F_FULLFSYNC.</p>

    <p>See also <a href="pragma.html#pragma_checkpoint_fullfsync">checkpoint_fullfsync</a>.</p>
<a name="pragma_function_list"></a>
<h _id=pragma_function_list style="display:none"> PRAGMA function_list</h><hr>
    <p><b>PRAGMA function_list;</b>
    <p>This pragma returns a list of SQL functions
    known to the database connection.

     <p style='background-color: #f0e0ff;'>
    This pragma is only available if SQLite is built using the
    <a href="compile.html#introspection_pragmas">-DSQLITE_INTROSPECTION_PRAGMAS</a> compile-time option.
  
<a name="pragma_ignore_check_constraints"></a>
<h _id=pragma_ignore_check_constraints style="display:none"> PRAGMA ignore_check_constraints</h><hr>
    <p><b>PRAGMA ignore_check_constraints  = </b><i>boolean</i><b>;</b></p>

    <p>This pragma enables or disables the enforcement of CHECK constraints.
    The default setting is off, meaning that CHECK constraints are
    enforced by default.</p>
................................................................................

    <p>The size of the memory-mapped I/O region cannot be changed while
       the memory-mapped I/O region is in active use, to avoid unmapping
       memory out from under running SQL statements.  For this reason,
       the mmap_size pragma may be a no-op if the prior mmap_size is non-zero
       and there are other SQL statements running concurrently on the same
       <a href="c3ref/sqlite3.html">database connection</a>.</p>
<a name="pragma_module_list"></a>
<h _id=pragma_module_list style="display:none"> PRAGMA module_list</h><hr>
    <p><b>PRAGMA module_list;</b>
    <p>This pragma returns a list of 
    <a href="vtab.html">virtual table</a> modules registered with the database connection.

     <p style='background-color: #f0e0ff;'>
    This pragma is only available if SQLite is built using the
    <a href="compile.html#introspection_pragmas">-DSQLITE_INTROSPECTION_PRAGMAS</a> compile-time option.
  
<a name="pragma_optimize"></a>
<h _id=pragma_optimize style="display:none"> PRAGMA optimize</h><hr>
  <p><b>PRAGMA optimize;
       <br>PRAGMA optimize(</b><i>MASK</i><b>);
       <br>PRAGMA </b><i>schema</i><b>.optimize;
       <br>PRAGMA </b><i>schema</i><b>.optimize(</b><i>MASK</i><b>);</b></p>
  <p>Attempt to optimize the database.  All schemas are optimized in the 
................................................................................

    
    <p style='background-color: #f0e0ff;'>
    This pragma is intended for use when debugging SQLite itself.  It
    is only available when the <a href="compile.html#debug">SQLITE_DEBUG</a> compile-time option
    is used.</p>
  
<a name="pragma_pragma_list"></a>
<h _id=pragma_pragma_list style="display:none"> PRAGMA pragma_list</h><hr>
    <p><b>PRAGMA pragma_list;</b>
    <p>This pragma returns a list of PRAGMA commands
    known to the database connection.

     <p style='background-color: #f0e0ff;'>
    This pragma is only available if SQLite is built using the
    <a href="compile.html#introspection_pragmas">-DSQLITE_INTROSPECTION_PRAGMAS</a> compile-time option.
  
<a name="pragma_query_only"></a>
<h _id=pragma_query_only style="display:none"> PRAGMA query_only</h><hr>
    <p><b>PRAGMA query_only;
      <br>PRAGMA query_only = </b><i>boolean</i><b>;</b></p>

    <p>The query_only pragma prevents all changes to database files when
    enabled.</p>

</p></dd><dt><b>argc &gt; 1 <br> argv[0] = NULL</b>
</dt><dd><p>
A new row is inserted with column values taken from
argv[2] and following.  In a rowid virtual table, if argv[1] is an SQL NULL,
then a new unique rowid is generated automatically.  The argv[1] will be NULL
for a <a href="vtab.html#worid">WITHOUT ROWID virtual table</a>, in which case the implementation should
take the PRIMARY KEY value from the appropiate column in argv[2] and following.

</p></dd><dt><b>argc &gt; 1 <br> argv[0] &ne; NULL <br> argv[0] = argv[1]</b>
</dt><dd><p>
The row with rowid or PRIMARY KEY argv[0] is updated with new values 
in argv[2] and following parameters.

</p></dd><dt><b>argc &gt; 1 <br> argv[0] &ne; NULL <br> argv[0] &ne; argv[1]</b>
................................................................................
using functions such as <a href="c3ref/mprintf.html">sqlite3_mprintf()</a> or <a href="c3ref/free.html">sqlite3_malloc()</a>.

</p><p>If the xUpdate method violates some constraint of the virtual table
(including, but not limited to, attempting to store a value of the wrong 
datatype, attempting to store a value that is too
large or too small, or attempting to change a read-only value) then the
xUpdate must fail with an appropriate <a href="rescode.html">error code</a>.




















</p><p>There might be one or more <a href="c3ref/vtab_cursor.html">sqlite3_vtab_cursor</a> objects open and in use 
on the virtual table instance and perhaps even on the row of the virtual
table when the xUpdate method is invoked.  The implementation of
xUpdate must be prepared for attempts to delete or modify rows of the table
out from other existing cursors.  If the virtual table cannot accommodate
such changes, the xUpdate method must return an <a href="rescode.html">error code</a>.

</p></dd><dt><b>argc &gt; 1 <br> argv[0] = NULL</b>
</dt><dd><p>
A new row is inserted with column values taken from
argv[2] and following.  In a rowid virtual table, if argv[1] is an SQL NULL,
then a new unique rowid is generated automatically.  The argv[1] will be NULL
for a <a href="vtab.html#worid">WITHOUT ROWID virtual table</a>, in which case the implementation should
take the PRIMARY KEY value from the appropriate column in argv[2] and following.

</p></dd><dt><b>argc &gt; 1 <br> argv[0] &ne; NULL <br> argv[0] = argv[1]</b>
</dt><dd><p>
The row with rowid or PRIMARY KEY argv[0] is updated with new values 
in argv[2] and following parameters.

</p></dd><dt><b>argc &gt; 1 <br> argv[0] &ne; NULL <br> argv[0] &ne; argv[1]</b>
................................................................................
using functions such as <a href="c3ref/mprintf.html">sqlite3_mprintf()</a> or <a href="c3ref/free.html">sqlite3_malloc()</a>.

</p><p>If the xUpdate method violates some constraint of the virtual table
(including, but not limited to, attempting to store a value of the wrong 
datatype, attempting to store a value that is too
large or too small, or attempting to change a read-only value) then the
xUpdate must fail with an appropriate <a href="rescode.html">error code</a>.

</p><p>If the xUpdate method is performing an UPDATE, then
<a href="c3ref/value_blob.html">sqlite3_value_nochange(X)</a> can be used to discover which columns
of the virtual table were actually modified by the UPDATE
statement.  The <a href="c3ref/value_blob.html">sqlite3_value_nochange(X)</a> interface returns
true for columns that do not change.
On every UPDATE, SQLite will first invoke
<a href="vtab.html#xcolumn">xColumn</a> separately for each unchanging column in the table to 
obtain the value for that column.  The <a href="vtab.html#xcolumn">xColumn</a> method can
check to see if the column is unchanged at the SQL level
by invoking <a href="c3ref/vtab_nochange.html">sqlite3_vtab_nochange()</a>.  If <a href="vtab.html#xcolumn">xColumn</a> sees that
the column is not being modified, it should return without setting 
a result using one of the <a href="c3ref/result_blob.html">sqlite3_result_xxxxx()</a>
interfaces.  Only in that case <a href="c3ref/value_blob.html">sqlite3_value_nochange()</a> will be
true within the xUpdate method.  If <a href="vtab.html#xcolumn">xColumn</a> does
invoke one or more <a href="c3ref/result_blob.html">sqlite3_result_xxxxx()</a>
interfaces, then SQLite understands that as a change in the value
of the column and the <a href="c3ref/value_blob.html">sqlite3_value_nochange()</a> call for that
column within xUpdate will return false.

</p><p>There might be one or more <a href="c3ref/vtab_cursor.html">sqlite3_vtab_cursor</a> objects open and in use 
on the virtual table instance and perhaps even on the row of the virtual
table when the xUpdate method is invoked.  The implementation of
xUpdate must be prepared for attempts to delete or modify rows of the table
out from other existing cursors.  If the virtual table cannot accommodate
such changes, the xUpdate method must return an <a href="rescode.html">error code</a>.

        /// No insert occurs.
        /// <![CDATA[</dd>]]><![CDATA[<dt>]]><![CDATA[<b>]]>argc &gt; 1 <![CDATA[<br>]]> argv[0] = NULL<![CDATA[</b>]]>
        /// <![CDATA[</dt>]]><![CDATA[<dd>]]>
        /// A new row is inserted with column values taken from
        /// argv[2] and following.  In a rowid virtual table, if argv[1] is an SQL NULL,
        /// then a new unique rowid is generated automatically.  The argv[1] will be NULL
        /// for a WITHOUT ROWID virtual table, in which case the implementation should
        /// take the PRIMARY KEY value from the appropiate column in argv[2] and following.
        /// <![CDATA[</dd>]]><![CDATA[<dt>]]><![CDATA[<b>]]>argc &gt; 1 <![CDATA[<br>]]> argv[0] &#8800; NULL <![CDATA[<br>]]> argv[0] = argv[1]<![CDATA[</b>]]>
        /// <![CDATA[</dt>]]><![CDATA[<dd>]]>
        /// The row with rowid or PRIMARY KEY argv[0] is updated with new values 
        /// in argv[2] and following parameters.
        /// <![CDATA[</dd>]]><![CDATA[<dt>]]><![CDATA[<b>]]>argc &gt; 1 <![CDATA[<br>]]> argv[0] &#8800; NULL <![CDATA[<br>]]> argv[0] &#8800; argv[1]<![CDATA[</b>]]>
        /// <![CDATA[</dt>]]><![CDATA[<dd>]]> The row with rowid or PRIMARY KEY argv[0] is updated with 
        /// the rowid or PRIMARY KEY in argv[1] 
................................................................................
        /// <para>
        /// If the xUpdate method violates some constraint of the virtual table
        /// (including, but not limited to, attempting to store a value of the wrong 
        /// datatype, attempting to store a value that is too
        /// large or too small, or attempting to change a read-only value) then the
        /// xUpdate must fail with an appropriate error code.
        /// </para>




















        /// <para>
        /// There might be one or more sqlite3_vtab_cursor objects open and in use 
        /// on the virtual table instance and perhaps even on the row of the virtual
        /// table when the xUpdate method is invoked.  The implementation of
        /// xUpdate must be prepared for attempts to delete or modify rows of the table
        /// out from other existing cursors.  If the virtual table cannot accommodate
        /// such changes, the xUpdate method must return an error code.

        /// No insert occurs.
        /// <![CDATA[</dd>]]><![CDATA[<dt>]]><![CDATA[<b>]]>argc &gt; 1 <![CDATA[<br>]]> argv[0] = NULL<![CDATA[</b>]]>
        /// <![CDATA[</dt>]]><![CDATA[<dd>]]>
        /// A new row is inserted with column values taken from
        /// argv[2] and following.  In a rowid virtual table, if argv[1] is an SQL NULL,
        /// then a new unique rowid is generated automatically.  The argv[1] will be NULL
        /// for a WITHOUT ROWID virtual table, in which case the implementation should
        /// take the PRIMARY KEY value from the appropriate column in argv[2] and following.
        /// <![CDATA[</dd>]]><![CDATA[<dt>]]><![CDATA[<b>]]>argc &gt; 1 <![CDATA[<br>]]> argv[0] &#8800; NULL <![CDATA[<br>]]> argv[0] = argv[1]<![CDATA[</b>]]>
        /// <![CDATA[</dt>]]><![CDATA[<dd>]]>
        /// The row with rowid or PRIMARY KEY argv[0] is updated with new values 
        /// in argv[2] and following parameters.
        /// <![CDATA[</dd>]]><![CDATA[<dt>]]><![CDATA[<b>]]>argc &gt; 1 <![CDATA[<br>]]> argv[0] &#8800; NULL <![CDATA[<br>]]> argv[0] &#8800; argv[1]<![CDATA[</b>]]>
        /// <![CDATA[</dt>]]><![CDATA[<dd>]]> The row with rowid or PRIMARY KEY argv[0] is updated with 
        /// the rowid or PRIMARY KEY in argv[1] 
................................................................................
        /// <para>
        /// If the xUpdate method violates some constraint of the virtual table
        /// (including, but not limited to, attempting to store a value of the wrong 
        /// datatype, attempting to store a value that is too
        /// large or too small, or attempting to change a read-only value) then the
        /// xUpdate must fail with an appropriate error code.
        /// </para>
        /// <para>
        /// If the xUpdate method is performing an UPDATE, then
        /// sqlite3_value_nochange(X) can be used to discover which columns
        /// of the virtual table were actually modified by the UPDATE
        /// statement.  The sqlite3_value_nochange(X) interface returns
        /// true for columns that do not change.
        /// On every UPDATE, SQLite will first invoke
        /// xColumn separately for each unchanging column in the table to 
        /// obtain the value for that column.  The xColumn method can
        /// check to see if the column is unchanged at the SQL level
        /// by invoking sqlite3_vtab_nochange().  If xColumn sees that
        /// the column is not being modified, it should return without setting 
        /// a result using one of the sqlite3_result_xxxxx()
        /// interfaces.  Only in that case sqlite3_value_nochange() will be
        /// true within the xUpdate method.  If xColumn does
        /// invoke one or more sqlite3_result_xxxxx()
        /// interfaces, then SQLite understands that as a change in the value
        /// of the column and the sqlite3_value_nochange() call for that
        /// column within xUpdate will return false.
        /// </para>
        /// <para>
        /// There might be one or more sqlite3_vtab_cursor objects open and in use 
        /// on the virtual table instance and perhaps even on the row of the virtual
        /// table when the xUpdate method is invoked.  The implementation of
        /// xUpdate must be prepared for attempts to delete or modify rows of the table
        /// out from other existing cursors.  If the virtual table cannot accommodate
        /// such changes, the xUpdate method must return an error code.