MXS-2397 Provide context information for fields

The query classifier now returns contextual information for a fields;
does it appear in the (right hand side) of a UNION or in a SUBQUERY.

include/maxscale/query_classifier.h

@@ -127,13 +127,23 @@ typedef enum qc_parse_result
 } qc_parse_result_t;
 
 /**
- * QC_FIELD_INFO contains information about a field used in a statement.
+ * qc_field_context_t defines the context where a field appears.
+ *
+ * NOTE: A particular bit does NOT mean that the field appears ONLY in the context,
+ *       but it may appear in other contexts as well.
  */
+typedef enum qc_field_context
+{
+    QC_FIELD_UNION    = 1, /** The field appears on the right hand side in a UNION. */
+    QC_FIELD_SUBQUERY = 2 /** The field appears in a subquery. */
+} qc_field_context_t;
+
 typedef struct qc_field_info
 {
-    char* database; /** Present if the field is of the form "a.b.c", NULL otherwise. */
-    char* table;    /** Present if the field is of the form "a.b", NULL otherwise. */
-    char* column;   /** Always present. */
+    char*    database; /** Present if the field is of the form "a.b.c", NULL otherwise. */
+    char*    table;    /** Present if the field is of the form "a.b", NULL otherwise. */
+    char*    column;   /** Always present. */
+    uint32_t context;  /** The context in which the field appears. */
 } QC_FIELD_INFO;
 
 /**