improve sql parser & wall filter

src/main/java/com/alibaba/druid/sql/dialect/mysql/parser/MySqlStatementParser.java

@@ -642,6 +642,7 @@ public class MySqlStatementParser extends SQLStatementParser {
         boolean full = false;
         if (lexer.token() == Token.FULL) {
             lexer.nextToken();
+            full = true;
         }
 
         if (identifierEquals("PROCESSLIST")) {
@@ -651,7 +652,7 @@ public class MySqlStatementParser extends SQLStatementParser {
             return stmt;
         }
 
-        if (identifierEquals("COLUMNS")) {
+        if (identifierEquals("COLUMNS") || identifierEquals("FIELDS")) {
             lexer.nextToken();
 
             MySqlShowColumnsStatement stmt = parseShowColumns();

src/main/java/com/alibaba/druid/sql/dialect/mysql/visitor/MySqlSchemaStatVisitor.java

@@ -422,7 +422,7 @@ public class MySqlSchemaStatVisitor extends SchemaStatVisitor implements MySqlAS
 
     @Override
     public boolean visit(MySqlShowColumnsStatement x) {
-        return true;
+        return false;
     }
 
     @Override
@@ -1283,7 +1283,7 @@ public class MySqlSchemaStatVisitor extends SchemaStatVisitor implements MySqlAS
     public boolean visit(MySqlAlterTableImportTablespace x) {
         return false;
     }
-
+    
     @Override
     public void endVisit(MySqlAlterTableImportTablespace x) {