Index: Makefile.in
==================================================================
--- Makefile.in
+++ Makefile.in
@@ -570,11 +570,13 @@
 TESTOPTS = --verbose=file --output=test-out.txt
 
 # Extra compiler options for various shell tools
 #
 SHELL_OPT = -DSQLITE_ENABLE_JSON1 -DSQLITE_ENABLE_FTS4
+# SHELL_OPT += -DSQLITE_ENABLE_FTS5
 SHELL_OPT += -DSQLITE_ENABLE_EXPLAIN_COMMENTS
+SHELL_OPT += -DSQLITE_ENABLE_UNKNOWN_SQL_FUNCTION
 FUZZERSHELL_OPT = -DSQLITE_ENABLE_JSON1
 FUZZCHECK_OPT = -DSQLITE_ENABLE_JSON1 -DSQLITE_ENABLE_MEMSYS5
 
 # This is the default Makefile target.  The objects listed here
 # are what get build when you type just "make" with no arguments.

Index: Makefile.msc
==================================================================
--- Makefile.msc
+++ Makefile.msc
@@ -1393,11 +1393,14 @@
 
 # Additional compiler options for the shell.  These are only effective
 # when the shell is not being dynamically linked.
 #
 !IF $(DYNAMIC_SHELL)==0 && $(FOR_WIN10)==0
-SHELL_COMPILE_OPTS = $(SHELL_COMPILE_OPTS) -DSQLITE_SHELL_JSON1 -DSQLITE_ENABLE_FTS4 -DSQLITE_ENABLE_EXPLAIN_COMMENTS
+SHELL_COMPILE_OPTS = $(SHELL_COMPILE_OPTS) -DSQLITE_SHELL_JSON1 -DSQLITE_ENABLE_FTS4
+# SHELL_COMPILE_OPTS = $(SHELL_COMPILE_OPTS) -DSQLITE_ENABLE_FTS5
+SHELL_COMPILE_OPTS = $(SHELL_COMPILE_OPTS) -DSQLITE_ENABLE_EXPLAIN_COMMENTS
+SHELL_COMPILE_OPTS = $(SHELL_COMPILE_OPTS) -DSQLITE_ENABLE_UNKNOWN_SQL_FUNCTION
 !ENDIF
 
 # <<mark>>
 # Extra compiler options for various test tools.
 #

Index: main.mk
==================================================================
--- main.mk
+++ main.mk
@@ -456,10 +456,12 @@
 TESTOPTS = --verbose=file --output=test-out.txt
 
 # Extra compiler options for various shell tools
 #
 SHELL_OPT = -DSQLITE_ENABLE_JSON1 -DSQLITE_ENABLE_FTS4 -DSQLITE_ENABLE_FTS5
+SHELL_OPT += -DSQLITE_ENABLE_EXPLAIN_COMMENTS
+SHELL_OPT += -DSQLITE_ENABLE_UNKNOWN_SQL_FUNCTION
 FUZZERSHELL_OPT = -DSQLITE_ENABLE_JSON1
 FUZZCHECK_OPT = -DSQLITE_ENABLE_JSON1 -DSQLITE_ENABLE_MEMSYS5
 
 # This is the default Makefile target.  The objects listed here
 # are what get build when you type just "make" with no arguments.

Index: src/expr.c
==================================================================
--- src/expr.c
+++ src/expr.c
@@ -2941,10 +2941,15 @@
       }
       nFarg = pFarg ? pFarg->nExpr : 0;
       assert( !ExprHasProperty(pExpr, EP_IntValue) );
       zId = pExpr->u.zToken;
       pDef = sqlite3FindFunction(db, zId, nFarg, enc, 0);
+#ifdef SQLITE_ENABLE_UNKNOWN_SQL_FUNCTION
+      if( pDef==0 && pParse->explain ){
+        pDef = sqlite3FindFunction(db, "unknown", nFarg, enc, 0);
+      }
+#endif
       if( pDef==0 || pDef->xFinalize!=0 ){
         sqlite3ErrorMsg(pParse, "unknown function: %s()", zId);
         break;
       }
 

Index: src/func.c
==================================================================
--- src/func.c
+++ src/func.c
@@ -1313,10 +1313,30 @@
     }
   }
   sqlite3_result_text(context, (char*)zIn, nIn, SQLITE_TRANSIENT);
 }
 
+
+#ifdef SQLITE_ENABLE_UNKNOWN_SQL_FUNCTION
+/*
+** The "unknown" function is automatically substituted in place of
+** any unrecognized function name when doing an EXPLAIN or EXPLAIN QUERY PLAN
+** when the SQLITE_ENABLE_UNKNOWN_FUNCTION compile-time option is used.
+** When the "sqlite3" command-line shell is built using this functionality,
+** that allows an EXPLAIN or EXPLAIN QUERY PLAN for complex queries
+** involving application-defined functions to be examined in a generic
+** sqlite3 shell.
+*/
+static void unknownFunc(
+  sqlite3_context *context,
+  int argc,
+  sqlite3_value **argv
+){
+  /* no-op */
+}
+#endif /*SQLITE_ENABLE_UNKNOWN_SQL_FUNCTION*/
+
 
 /* IMP: R-25361-16150 This function is omitted from SQLite by default. It
 ** is only available if the SQLITE_SOUNDEX compile-time option is used
 ** when SQLite is built.
 */
@@ -1784,17 +1804,20 @@
     AGGREGATE(count,             1, 0, 0, countStep,       countFinalize  ),
     AGGREGATE(group_concat,      1, 0, 0, groupConcatStep, groupConcatFinalize),
     AGGREGATE(group_concat,      2, 0, 0, groupConcatStep, groupConcatFinalize),
   
     LIKEFUNC(glob, 2, &globInfo, SQLITE_FUNC_LIKE|SQLITE_FUNC_CASE),
-  #ifdef SQLITE_CASE_SENSITIVE_LIKE
+#ifdef SQLITE_CASE_SENSITIVE_LIKE
     LIKEFUNC(like, 2, &likeInfoAlt, SQLITE_FUNC_LIKE|SQLITE_FUNC_CASE),
     LIKEFUNC(like, 3, &likeInfoAlt, SQLITE_FUNC_LIKE|SQLITE_FUNC_CASE),
-  #else
+#else
     LIKEFUNC(like, 2, &likeInfoNorm, SQLITE_FUNC_LIKE),
     LIKEFUNC(like, 3, &likeInfoNorm, SQLITE_FUNC_LIKE),
-  #endif
+#endif
+#ifdef SQLITE_ENABLE_UNKNOWN_SQL_FUNCTION
+    FUNCTION(unknown,           -1, 0, 0, unknownFunc      ),
+#endif
     FUNCTION(coalesce,           1, 0, 0, 0                ),
     FUNCTION(coalesce,           0, 0, 0, 0                ),
     FUNCTION2(coalesce,         -1, 0, 0, noopFunc,  SQLITE_FUNC_COALESCE),
   };
 #ifndef SQLITE_OMIT_ALTERTABLE

Index: src/resolve.c
==================================================================
--- src/resolve.c
+++ src/resolve.c
@@ -716,11 +716,15 @@
       }
       if( is_agg && (pNC->ncFlags & NC_AllowAgg)==0 ){
         sqlite3ErrorMsg(pParse, "misuse of aggregate function %.*s()", nId,zId);
         pNC->nErr++;
         is_agg = 0;
-      }else if( no_such_func && pParse->db->init.busy==0 ){
+      }else if( no_such_func && pParse->db->init.busy==0
+#ifdef SQLITE_ENABLE_UNKNOWN_SQL_FUNCTION
+                && pParse->explain==0
+#endif
+      ){
         sqlite3ErrorMsg(pParse, "no such function: %.*s", nId, zId);
         pNC->nErr++;
       }else if( wrong_num_args ){
         sqlite3ErrorMsg(pParse,"wrong number of arguments to function %.*s()",
              nId, zId);