Add the experimental SQLITE_ENABLE_UNKNOWN_SQL_FUNCTION compile-time option.

When enabled, the "unknown function" error is suppressed for EXPLAIN and
a no-op function named "unknown()" is substituted.  This facilitiates using
the command-line shell to analyze queries from applications that contain
many application-defined functions that are not normally available to the
shell.

FossilOrigin-Name: b7f30a9ff20d580fdaecdcf2b644d09ad6c2575e

src/func.c

@@ -1316,6 +1316,26 @@ static void trimFunc(
 }
 
 
+#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.
@@ -1786,13 +1806,16 @@ void sqlite3RegisterBuiltinFunctions(void){
     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),