diff --git a/Makefile.in b/Makefile.in
index f84dcc25ee..96e2eb11f8 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -437,6 +437,7 @@ TESTSRC += \
   $(TOP)/ext/misc/closure.c \
   $(TOP)/ext/misc/csv.c \
   $(TOP)/ext/misc/eval.c \
+  $(TOP)/ext/misc/explain.c \
   $(TOP)/ext/misc/fileio.c \
   $(TOP)/ext/misc/fuzzer.c \
   $(TOP)/ext/fts5/fts5_tcl.c \
diff --git a/Makefile.msc b/Makefile.msc
index 4c6cdfba1d..b9fab7c408 100644
--- a/Makefile.msc
+++ b/Makefile.msc
@@ -1515,6 +1515,7 @@ TESTEXT = \
   $(TOP)\ext\misc\closure.c \
   $(TOP)\ext\misc\csv.c \
   $(TOP)\ext\misc\eval.c \
+  $(TOP)\ext\misc\explain.c \
   $(TOP)\ext\misc\fileio.c \
   $(TOP)\ext\misc\fuzzer.c \
   $(TOP)\ext\fts5\fts5_tcl.c \
diff --git a/ext/misc/explain.c b/ext/misc/explain.c
new file mode 100644
index 0000000000..c2c8c07a5c
--- /dev/null
+++ b/ext/misc/explain.c
@@ -0,0 +1,308 @@
+/*
+** 2018-09-16
+**
+** The author disclaims copyright to this source code.  In place of
+** a legal notice, here is a blessing:
+**
+**    May you do good and not evil.
+**    May you find forgiveness for yourself and forgive others.
+**    May you share freely, never taking more than you give.
+**
+*************************************************************************
+**
+** This file demonstrates an eponymous virtual table that returns the
+** EXPLAIN output from an SQL statement.
+**
+** Usage example:
+**
+**     .load ./explain
+**     SELECT p2 FROM explain('SELECT * FROM sqlite_master')
+**      WHERE opcode='OpenRead';
+*/
+#if !defined(SQLITEINT_H)
+#include "sqlite3ext.h"
+#endif
+SQLITE_EXTENSION_INIT1
+#include <assert.h>
+#include <string.h>
+
+#ifndef SQLITE_OMIT_VIRTUALTABLE
+
+/* explain_vtab is a subclass of sqlite3_vtab which will
+** serve as the underlying representation of a explain virtual table
+*/
+typedef struct explain_vtab explain_vtab;
+struct explain_vtab {
+  sqlite3_vtab base;  /* Base class - must be first */
+  sqlite3 *db;        /* Database connection for this explain vtab */
+};
+
+/* explain_cursor is a subclass of sqlite3_vtab_cursor which will
+** serve as the underlying representation of a cursor that scans
+** over rows of the result from an EXPLAIN operation.
+*/
+typedef struct explain_cursor explain_cursor;
+struct explain_cursor {
+  sqlite3_vtab_cursor base;  /* Base class - must be first */
+  sqlite3 *db;               /* Database connection for this cursor */
+  char *zSql;                /* Value for the EXPLN_COLUMN_SQL column */
+  sqlite3_stmt *pExplain;    /* Statement being explained */
+  int rc;                    /* Result of last sqlite3_step() on pExplain */
+};
+
+/*
+** The explainConnect() method is invoked to create a new
+** explain_vtab that describes the explain virtual table.
+**
+** Think of this routine as the constructor for explain_vtab objects.
+**
+** All this routine needs to do is:
+**
+**    (1) Allocate the explain_vtab object and initialize all fields.
+**
+**    (2) Tell SQLite (via the sqlite3_declare_vtab() interface) what the
+**        result set of queries against explain will look like.
+*/
+static int explainConnect(
+  sqlite3 *db,
+  void *pAux,
+  int argc, const char *const*argv,
+  sqlite3_vtab **ppVtab,
+  char **pzErr
+){
+  explain_vtab *pNew;
+  int rc;
+
+/* Column numbers */
+#define EXPLN_COLUMN_ADDR     0   /* Instruction address */
+#define EXPLN_COLUMN_OPCODE   1   /* Opcode */
+#define EXPLN_COLUMN_P1       2   /* Operand 1 */
+#define EXPLN_COLUMN_P2       3   /* Operand 2 */
+#define EXPLN_COLUMN_P3       4   /* Operand 3 */
+#define EXPLN_COLUMN_P4       5   /* Operand 4 */
+#define EXPLN_COLUMN_P5       6   /* Operand 5 */
+#define EXPLN_COLUMN_COMMENT  7   /* Comment */
+#define EXPLN_COLUMN_SQL      8   /* SQL that is being explained */
+
+
+  rc = sqlite3_declare_vtab(db,
+     "CREATE TABLE x(addr,opcode,p1,p2,p3,p4,p5,comment,sql HIDDEN)");
+  if( rc==SQLITE_OK ){
+    pNew = sqlite3_malloc( sizeof(*pNew) );
+    *ppVtab = (sqlite3_vtab*)pNew;
+    if( pNew==0 ) return SQLITE_NOMEM;
+    memset(pNew, 0, sizeof(*pNew));
+    pNew->db = db;
+  }
+  return rc;
+}
+
+/*
+** This method is the destructor for explain_cursor objects.
+*/
+static int explainDisconnect(sqlite3_vtab *pVtab){
+  sqlite3_free(pVtab);
+  return SQLITE_OK;
+}
+
+/*
+** Constructor for a new explain_cursor object.
+*/
+static int explainOpen(sqlite3_vtab *p, sqlite3_vtab_cursor **ppCursor){
+  explain_cursor *pCur;
+  pCur = sqlite3_malloc( sizeof(*pCur) );
+  if( pCur==0 ) return SQLITE_NOMEM;
+  memset(pCur, 0, sizeof(*pCur));
+  pCur->db = ((explain_vtab*)p)->db;
+  *ppCursor = &pCur->base;
+  return SQLITE_OK;
+}
+
+/*
+** Destructor for a explain_cursor.
+*/
+static int explainClose(sqlite3_vtab_cursor *cur){
+  explain_cursor *pCur = (explain_cursor*)cur;
+  sqlite3_finalize(pCur->pExplain);
+  sqlite3_free(pCur->zSql);
+  sqlite3_free(pCur);
+  return SQLITE_OK;
+}
+
+
+/*
+** Advance a explain_cursor to its next row of output.
+*/
+static int explainNext(sqlite3_vtab_cursor *cur){
+  explain_cursor *pCur = (explain_cursor*)cur;
+  pCur->rc = sqlite3_step(pCur->pExplain);
+  if( pCur->rc!=SQLITE_DONE && pCur->rc!=SQLITE_ROW ) return pCur->rc;
+  return SQLITE_OK;
+}
+
+/*
+** Return values of columns for the row at which the explain_cursor
+** is currently pointing.
+*/
+static int explainColumn(
+  sqlite3_vtab_cursor *cur,   /* The cursor */
+  sqlite3_context *ctx,       /* First argument to sqlite3_result_...() */
+  int i                       /* Which column to return */
+){
+  explain_cursor *pCur = (explain_cursor*)cur;
+  if( i==EXPLN_COLUMN_SQL ){
+    sqlite3_result_text(ctx, pCur->zSql, -1, SQLITE_TRANSIENT);
+  }else{
+    sqlite3_result_value(ctx, sqlite3_column_value(pCur->pExplain, i));
+  }
+  return SQLITE_OK;
+}
+
+/*
+** Return the rowid for the current row.  In this implementation, the
+** rowid is the same as the output value.
+*/
+static int explainRowid(sqlite3_vtab_cursor *cur, sqlite_int64 *pRowid){
+  explain_cursor *pCur = (explain_cursor*)cur;
+  *pRowid = sqlite3_column_int64(pCur->pExplain, 0);
+  return SQLITE_OK;
+}
+
+/*
+** Return TRUE if the cursor has been moved off of the last
+** row of output.
+*/
+static int explainEof(sqlite3_vtab_cursor *cur){
+  explain_cursor *pCur = (explain_cursor*)cur;
+  return pCur->rc!=SQLITE_ROW;
+}
+
+/*
+** This method is called to "rewind" the explain_cursor object back
+** to the first row of output.  This method is always called at least
+** once prior to any call to explainColumn() or explainRowid() or 
+** explainEof().
+**
+** The argv[0] is the SQL statement that is to be explained.
+*/
+static int explainFilter(
+  sqlite3_vtab_cursor *pVtabCursor, 
+  int idxNum, const char *idxStr,
+  int argc, sqlite3_value **argv
+){
+  explain_cursor *pCur = (explain_cursor *)pVtabCursor;
+  char *zSql = 0;
+  int rc;
+  sqlite3_finalize(pCur->pExplain);
+  pCur->pExplain = 0;
+  if( sqlite3_value_type(argv[0])!=SQLITE_TEXT ){
+    pCur->rc = SQLITE_DONE;
+    return SQLITE_OK;
+  }
+  sqlite3_free(pCur->zSql);
+  pCur->zSql = sqlite3_mprintf("%s", sqlite3_value_text(argv[0]));
+  if( pCur->zSql ){
+    zSql = sqlite3_mprintf("EXPLAIN %s", pCur->zSql);
+  }
+  if( zSql==0 ){
+    rc = SQLITE_NOMEM;
+  }else{
+    rc = sqlite3_prepare_v2(pCur->db, zSql, -1, &pCur->pExplain, 0);
+  }
+  if( rc ){
+    sqlite3_finalize(pCur->pExplain);
+    pCur->pExplain = 0;
+    sqlite3_free(pCur->zSql);
+    pCur->zSql = 0;
+  }else{
+    pCur->rc = sqlite3_step(pCur->pExplain);
+    rc = (pCur->rc==SQLITE_DONE || pCur->rc==SQLITE_ROW) ? SQLITE_OK : pCur->rc;
+  }
+  return rc;
+}
+
+/*
+** SQLite will invoke this method one or more times while planning a query
+** that uses the explain virtual table.  This routine needs to create
+** a query plan for each invocation and compute an estimated cost for that
+** plan.
+*/
+static int explainBestIndex(
+  sqlite3_vtab *tab,
+  sqlite3_index_info *pIdxInfo
+){
+  int i;
+
+  pIdxInfo->estimatedCost = (double)1000000;
+  pIdxInfo->estimatedRows = 500;
+  for(i=0; i<pIdxInfo->nConstraint; i++){
+    struct sqlite3_index_constraint *p = &pIdxInfo->aConstraint[i];
+    if( p->usable
+     && p->iColumn==EXPLN_COLUMN_SQL
+     && p->op==SQLITE_INDEX_CONSTRAINT_EQ
+    ){
+      pIdxInfo->estimatedCost = 10.0;
+      pIdxInfo->idxNum = 1;
+      pIdxInfo->aConstraintUsage[i].argvIndex = 1;
+      pIdxInfo->aConstraintUsage[i].omit = 1;
+      break;
+    }
+  }
+  return SQLITE_OK;
+}
+
+/*
+** This following structure defines all the methods for the 
+** explain virtual table.
+*/
+static sqlite3_module explainModule = {
+  0,                         /* iVersion */
+  0,                         /* xCreate */
+  explainConnect,            /* xConnect */
+  explainBestIndex,          /* xBestIndex */
+  explainDisconnect,         /* xDisconnect */
+  0,                         /* xDestroy */
+  explainOpen,               /* xOpen - open a cursor */
+  explainClose,              /* xClose - close a cursor */
+  explainFilter,             /* xFilter - configure scan constraints */
+  explainNext,               /* xNext - advance a cursor */
+  explainEof,                /* xEof - check for end of scan */
+  explainColumn,             /* xColumn - read data */
+  explainRowid,              /* xRowid - read data */
+  0,                         /* xUpdate */
+  0,                         /* xBegin */
+  0,                         /* xSync */
+  0,                         /* xCommit */
+  0,                         /* xRollback */
+  0,                         /* xFindMethod */
+  0,                         /* xRename */
+  0,                         /* xSavepoint */
+  0,                         /* xRelease */
+  0,                         /* xRollbackTo */
+};
+
+#endif /* SQLITE_OMIT_VIRTUALTABLE */
+
+int sqlite3ExplainVtabInit(sqlite3 *db){
+  int rc = SQLITE_OK;
+#ifndef SQLITE_OMIT_VIRTUALTABLE
+  rc = sqlite3_create_module(db, "explain", &explainModule, 0);
+#endif
+  return rc;
+}
+
+#ifdef _WIN32
+__declspec(dllexport)
+#endif
+int sqlite3_explain_init(
+  sqlite3 *db, 
+  char **pzErrMsg, 
+  const sqlite3_api_routines *pApi
+){
+  int rc = SQLITE_OK;
+  SQLITE_EXTENSION_INIT2(pApi);
+#ifndef SQLITE_OMIT_VIRTUALTABLE
+  rc = sqlite3ExplainVtabInit(db);
+#endif
+  return rc;
+}
diff --git a/main.mk b/main.mk
index f32624dfd3..1df68076c1 100644
--- a/main.mk
+++ b/main.mk
@@ -361,6 +361,7 @@ TESTSRC += \
   $(TOP)/ext/misc/closure.c \
   $(TOP)/ext/misc/csv.c \
   $(TOP)/ext/misc/eval.c \
+  $(TOP)/ext/misc/explain.c \
   $(TOP)/ext/misc/fileio.c \
   $(TOP)/ext/misc/fuzzer.c \
   $(TOP)/ext/misc/ieee754.c \
diff --git a/manifest b/manifest
index 8eb0bd6afc..015c3618f7 100644
--- a/manifest
+++ b/manifest
@@ -1,10 +1,10 @@
-C Improved\spresentation\son\sthe\snew\scode\sthat\sprevents\sunnecessary\swrites\sto\nexpressions\son\sindexes\sduring\san\sUPDATE\swhen\sthe\sexpression\sdoes\snot\sreference\nany\sof\sthe\scolumns\sthat\sare\schanging.
-D 2018-09-16T15:01:25.994
+C Add\sthe\snew\s"explain"\svirtual\stable\sin\sext/misc.\s\sUse\sthis\svirtual\stable\nfor\sadditional\stest\scases\sfor\sthe\soptimization\sthat\savoids\supdating\sindexes\non\sexpressions\swhen\snone\sof\sthe\scolumns\schanged\sby\sthe\sUPDATE\sare\sin\sthe\nexpression.
+D 2018-09-16T16:18:01.470
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
-F Makefile.in 6b650013511fd9d8b094203ac268af9220d292cc7d4e1bc9fbca15aacd8c7995
+F Makefile.in 01e95208a78b57d056131382c493c963518f36da4c42b12a97eb324401b3a334
 F Makefile.linux-gcc 7bc79876b875010e8c8f9502eb935ca92aa3c434
-F Makefile.msc a889c6981a222d639f8a548adcfb3183ac07871e27452ace4d810735750f216a
+F Makefile.msc b946f8806a5d401a299453f61de80dfd1a9df14fa4902b299e6465e3c3134872
 F README.md 377233394b905d3b2e2b33741289e093bc93f2e7adbe00923b2c5958c9a9edee
 F VERSION 654da1d4053fb09ffc33a3910e6d427182a7dcdc67e934fa83de2849ac83fccb
 F aclocal.m4 a5c22d164aff7ed549d53a90fa56d56955281f50
@@ -281,6 +281,7 @@ F ext/misc/compress.c dd4f8a6d0baccff3c694757db5b430f3bbd821d8686d1fc24df55cf9f0
 F ext/misc/csv.c 65297bcce8d5acd5aadef42acbe739aef5a2ef5e74c7b73361ca19f3e21de657
 F ext/misc/dbdump.c 12389a10c410fadf1e68eeb382def92d5a7fa9ce7cce4fb86a736fa2bac1000a
 F ext/misc/eval.c 6ea9b22a5fa0dd973b67ca4e53555be177bc0b7b263aadf1024429457c82c0e3
+F ext/misc/explain.c 657b95855157379b406af602ed0e1cf5d136c0947cdd207fa5470797c8a1ce18
 F ext/misc/fileio.c 7317d825fab6a3c48f6e3822a00a6a22e08e55af31700ac96f16a523f83069fd
 F ext/misc/fuzzer.c 7c64b8197bb77b7d64eff7cac7848870235d4c25
 F ext/misc/ieee754.c f190d0cc5182529acb15babd177781be1ac1718c
@@ -423,7 +424,7 @@ F ext/userauth/userauth.c 3410be31283abba70255d71fd24734e017a4497f
 F install-sh 9d4de14ab9fb0facae2f48780b874848cbf2f895 x
 F ltmain.sh 3ff0879076df340d2e23ae905484d8c15d5fdea8
 F magic.txt 8273bf49ba3b0c8559cb2774495390c31fd61c60
-F main.mk 1db6df4bff24ed6684917e3fe311ce28f5924d6417c698fe4326f7cadf02df31
+F main.mk ff82d38126f8f0668b7990e0f1f3dcd74fa2d477c19b2e3feaaba586051e9b48
 F mkso.sh fd21c06b063bb16a5d25deea1752c2da6ac3ed83
 F mptest/config01.test 3c6adcbc50b991866855f1977ff172eb6d901271
 F mptest/config02.test 4415dfe36c48785f751e16e32c20b077c28ae504
@@ -511,7 +512,7 @@ F src/sqliteLimit.h 1513bfb7b20378aa0041e7022d04acb73525de35b80b252f1b83fedb4de6
 F src/status.c 46e7aec11f79dad50965a5ca5fa9de009f7d6bde08be2156f1538a0a296d4d0e
 F src/table.c b46ad567748f24a326d9de40e5b9659f96ffff34
 F src/tclsqlite.c e0bf71a6d24b8c23393c000abffab05979bbca2a72d0b0f79260e2cf1527fda5
-F src/test1.c 31c491ccb536bd9916a084e732ffe783b3c8973f2586d5a56aed0e3a9701dfff
+F src/test1.c 9bb042e4afedc570f78638993fc9cc1760d897d3b27dd72c20618044b2a8fa5e
 F src/test2.c 3efb99ab7f1fc8d154933e02ae1378bac9637da5
 F src/test3.c 61798bb0d38b915067a8c8e03f5a534b431181f802659a6616f9b4ff7d872644
 F src/test4.c 18ec393bb4d0ad1de729f0b94da7267270f3d8e6
@@ -1007,7 +1008,7 @@ F test/index8.test bc2e3db70e8e62459aaa1bd7e4a9b39664f8f9d7
 F test/index9.test 0aa3e509dddf81f93380396e40e9bb386904c1054924ba8fa9bcdfe85a8e7721
 F test/indexedby.test a52c8c6abfae4fbfb51d99440de4ca1840dbacc606b05e29328a2a8ba7cd914e
 F test/indexexpr1.test 635261197bcdc19b9b2c59bbfa7227d525c00e9587faddb2d293c44d287ce60e
-F test/indexexpr2.test 05f490de32b07bce041947cb63475eb04cbd5af2ea2979ca6b9a694cde176efd
+F test/indexexpr2.test d8f56b6cf4e5663486129bc18c6bde4cef9f8876942b001bfdd692a2b1a42668
 F test/indexfault.test 31d4ab9a7d2f6e9616933eb079722362a883eb1d
 F test/init.test 15c823093fdabbf7b531fe22cf037134d09587a7
 F test/insert.test 5604b1ff5675cc84c34a5b315792b958f48c32edccc0dafcc81d3b776270b70a
@@ -1765,7 +1766,7 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P 885f0f8252aae776a86c64bcc7da582f0ed58f2caae8ebff810a83ca339da820
-R 1d15143421504697e5d8ab90a5ba9295
+P c9f045295c4577752b0847ff2027b44661e6cb15bb08b942ccb3a0ef396f3dec
+R 58a4dfba8e02bbbb06c952d22469b1fa
 U drh
-Z 8e73db792f1c5d12613e11c54c1867c6
+Z ae02262bf6d409f9ac69a0871114af64
diff --git a/manifest.uuid b/manifest.uuid
index ee1b06ef03..1c4ba86291 100644
--- a/manifest.uuid
+++ b/manifest.uuid
@@ -1 +1 @@
-c9f045295c4577752b0847ff2027b44661e6cb15bb08b942ccb3a0ef396f3dec
\ No newline at end of file
+2404304cc15eaeee2744cf3c8f9cac0a544631c4f1060c5a17a78b43ca86edf0
\ No newline at end of file
diff --git a/src/test1.c b/src/test1.c
index 7f301714bd..d2c997bdfe 100644
--- a/src/test1.c
+++ b/src/test1.c
@@ -7038,6 +7038,7 @@ static int SQLITE_TCLAPI tclLoadStaticExtensionCmd(
   extern int sqlite3_closure_init(sqlite3*,char**,const sqlite3_api_routines*);
   extern int sqlite3_csv_init(sqlite3*,char**,const sqlite3_api_routines*);
   extern int sqlite3_eval_init(sqlite3*,char**,const sqlite3_api_routines*);
+  extern int sqlite3_explain_init(sqlite3*,char**,const sqlite3_api_routines*);
   extern int sqlite3_fileio_init(sqlite3*,char**,const sqlite3_api_routines*);
   extern int sqlite3_fuzzer_init(sqlite3*,char**,const sqlite3_api_routines*);
   extern int sqlite3_ieee_init(sqlite3*,char**,const sqlite3_api_routines*);
@@ -7062,6 +7063,7 @@ static int SQLITE_TCLAPI tclLoadStaticExtensionCmd(
     { "closure",               sqlite3_closure_init              },
     { "csv",                   sqlite3_csv_init                  },
     { "eval",                  sqlite3_eval_init                 },
+    { "explain",               sqlite3_explain_init              },
     { "fileio",                sqlite3_fileio_init               },
     { "fuzzer",                sqlite3_fuzzer_init               },
     { "ieee754",               sqlite3_ieee_init                 },
diff --git a/test/indexexpr2.test b/test/indexexpr2.test
index 856992201e..3fa3faf87a 100644
--- a/test/indexexpr2.test
+++ b/test/indexexpr2.test
@@ -198,4 +198,36 @@ do_test 4.130 {
   # Refcnt() should not be invoked because that index does not change.
 } {0}
 
+# Additional test cases to show that UPDATE does not modify indexes that
+# do not involve unchanged columns.
+#
+load_static_extension db explain
+do_execsql_test 4.200 {
+  CREATE TABLE t2(a,b,c,d,e,f);
+  INSERT INTO t2 VALUES(2,3,4,5,6,7);
+  CREATE INDEX t2abc ON t2(a+b+c);
+  CREATE INDEX t2cd ON t2(c*d);
+  CREATE INDEX t2def ON t2(d,e+25*f);
+  SELECT sqlite_master.name 
+    FROM sqlite_master, explain('UPDATE t2 SET b=b+1')
+   WHERE explain.opcode LIKE 'Open%'
+     AND sqlite_master.rootpage=explain.p2
+   ORDER BY 1;
+} {t2 t2abc}
+do_execsql_test 4.210 {
+  SELECT sqlite_master.name 
+    FROM sqlite_master, explain('UPDATE t2 SET c=c+1')
+   WHERE explain.opcode LIKE 'Open%'
+     AND sqlite_master.rootpage=explain.p2
+   ORDER BY 1;
+} {t2 t2abc t2cd}
+do_execsql_test 4.220 {
+  SELECT sqlite_master.name 
+    FROM sqlite_master, explain('UPDATE t2 SET c=c+1, f=NULL')
+   WHERE explain.opcode LIKE 'Open%'
+     AND sqlite_master.rootpage=explain.p2
+   ORDER BY 1;
+} {t2 t2abc t2cd t2def}
+
+
 finish_test