Add the sqlite3_value_nochange() API, usable from within the xUpdate method

of a virtual table to discover whether or not a column was unchanged at the
SQL level.

FossilOrigin-Name: dec3ea4e4e6c4b1761ddc883a29eaa50dcd663ce6199667cc0ff82f7849d4f2a

diff --cc manifest
index d9b6cfb745c2ba5682a32995559234acabeea32d,06de123c8ea7ec9c3185dc204d4642592dc31863..8932dc15adde123b1db11d61fee1f450bdd67949
--- 1/manifest
--- 2/manifest
+++ b/manifest
@@@ -1,5 -1,5 +1,5 @@@
- C Simplification\sto\sthe\simplementation\sof\sOP_Insert.
- D 2018-01-12T19:33:18.062
 -C Improved\scomments.\s\sSlightly\stighter\simplementation,\sbut\sno\sbig\schanges.
 -D 2018-01-12T23:18:38.948
++C Add\sthe\ssqlite3_value_nochange()\sAPI,\susable\sfrom\swithin\sthe\sxUpdate\smethod\nof\sa\svirtual\stable\sto\sdiscover\swhether\sor\snot\sa\scolumn\swas\sunchanged\sat\sthe\nSQL\slevel.
++D 2018-01-12T23:38:10.473
  F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
  F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
  F Makefile.in 38f84f301cbef443b2d269f67a74b8cc536469831f70df7c3e912acc04932cc2
@@@ -487,10 -487,10 +487,10 @@@ F src/resolve.c bbee7e31d369a18a2f48366
  F src/rowset.c 7b7e7e479212e65b723bf40128c7b36dc5afdfac
  F src/select.c 8b22abe193e4d8243befa2038e4ae2405802fed1c446e5e502d11f652e09ba74
  F src/shell.c.in b87abffd0db09203ad8a133d56fe8f154ace5ec0a14197a153fb7d80b1438c01
- F src/sqlite.h.in f83e63a48fb31fefc69c83bbe8700b4b44acdd64e440219087b0f14e35eeb8d4
 -F src/sqlite.h.in 86cc0db2b9a55ae7bbde80c9387cdb56aabe24221ee69069ec09626493cff9a8
++F src/sqlite.h.in 9daf78e8f3cecc9ea0c3a82201f75bb74f789ecbfcda28d2e47fa80b3d956961
  F src/sqlite3.rc 5121c9e10c3964d5755191c80dd1180c122fc3a8
  F src/sqlite3ext.h c02d628cca67f3889c689d82d25c3eb45e2c155db08e4c6089b5840d64687d34
- F src/sqliteInt.h fd8702c65994d7de3e2d8f7d85d958731da1ed29476571fdfa2290fd8ec0bf80
+ F src/sqliteInt.h 9c70315598b34810a83e4894455acb18e95cf63ce4e6cbb451ac2d17eabc2544
  F src/sqliteLimit.h 1513bfb7b20378aa0041e7022d04acb73525de35b80b252f1b83fedb4de6a76b
  F src/status.c 9737ed017279a9e0c5da748701c3c7bf1e8ae0dae459aad20dd64fcff97a7e35
  F src/table.c b46ad567748f24a326d9de40e5b9659f96ffff34
@@@ -1699,7 -1699,7 +1699,8 @@@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a9
  F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
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  F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
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 -Z 1d40e26fdb71cc6674c4c60bc100f971
++Z 1fe0c448d0c954d66825b437c30d556c

diff --cc manifest.uuid
index cf6d922bb8396452f21d1af675386ad767c4f392,20282b71748753e484effaa1b6444aff0f44eff3..39a147bce299aa4b8911d9ef7cb69dad6a866077
--- 1/manifest.uuid
--- 2/manifest.uuid
+++ b/manifest.uuid
@@@ -1,1 -1,1 +1,1 @@@
- 6acbdba59e9df4313a6232d925a70390acdc43dfa380b4fba7bb8bd442d6e728
 -a1b3f28569f2a8d82b2931527fdfe191b421f3e1ea18ee30e04211e1ad645993
++dec3ea4e4e6c4b1761ddc883a29eaa50dcd663ce6199667cc0ff82f7849d4f2a

diff --cc src/sqlite.h.in
index ba673748ed2dbaaba6993e6911d3aa3d76fcc3d9,cd524a046f94581b6b5a56af45b3e461f5c8925b..2661c8f35f07d7a774b9381781680ac48d1e79cc
--- 1/src/sqlite.h.in
--- 2/src/sqlite.h.in
+++ b/src/sqlite.h.in
@@@ -4799,6 -4799,6 +4799,9 @@@ SQLITE_DEPRECATED int sqlite3_memory_al
  ** datatype of the value
  ** <tr><td><b>sqlite3_value_numeric_type&nbsp;&nbsp;</b>
  ** <td>&rarr;&nbsp;&nbsp;<td>Best numeric datatype of the value
++** <tr><td><b>sqlite3_value_nochange&nbsp;&nbsp;</b>
++** <td>&rarr;&nbsp;&nbsp;<td>True if the column is unchanged in an UPDATE
++** against a virtual table.
  ** </table></blockquote>
  **
  ** <b>Details:</b>
@@@ -4847,6 -4847,17 +4850,19 @@@
  ** then the conversion is performed.  Otherwise no conversion occurs.
  ** The [SQLITE_INTEGER | datatype] after conversion is returned.)^
  **
 -** was unchanging).  If sqlite3_value_nochange(X) is invoked anywhere other
+ ** ^Within the [xUpdate] method of a [virtual table], the
+ ** sqlite3_value_nochange(X) interface returns true if and only if
+ ** the column corresponding to X is unchanged by the UPDATE operation
+ ** that the xUpdate method call was invoked to implement and if
+ ** and the prior [xColumn] method call that was invoked to extracted
+ ** the value for that column returned without setting a result (probably
+ ** because it queried [sqlite3_vtab_nochange()] and found that the column
++** was unchanging).  ^Within an [xUpdate] method, any value for which
++** sqlite3_value_nochange(X) is true will in all other respects appear
++** to be a NULL value.  If sqlite3_value_nochange(X) is invoked anywhere other
+ ** than within an [xUpdate] method call for an UPDATE statement, then
+ ** the return value is arbitrary and meaningless.
+ **
  ** Please pay particular attention to the fact that the pointer returned
  ** from [sqlite3_value_blob()], [sqlite3_value_text()], or
  ** [sqlite3_value_text16()] can be invalidated by a subsequent call to
@@@ -8306,6 -8318,6 +8323,13 @@@ int sqlite3_vtab_on_conflict(sqlite3 *)
  ** column value will not change.  Applications might use this to substitute
  ** a lighter-weight value to return that the corresponding [xUpdate] method
  ** understands as a "no-change" value.
++**
++** If the [xColumn] method calls sqlite3_vtab_nochange() and finds that
++** the column is not changed by the UPDATE statement, they the xColumn
++** method can optionally return without setting a result, without calling
++** any of the [sqlite3_result_int|sqlite3_result_xxxxx() interfaces].
++** In that case, [sqlite3_value_nochange(X)] will return true for the
++** same column in the [xUpdate] method.
  */
  int sqlite3_vtab_nochange(sqlite3_context*);