[enhancement](mysql) Add have_query_cache variable to be compatible with old mysql client (#21701)

docs/en/docs/advanced/variables.md

@@ -177,6 +177,10 @@ Note that the comment must start with /*+ and can only follow the SELECT.
 
     Used for compatibility with MySQL clients. No practical effect.
 
+* `have_query_cache`
+
+  Used for compatibility with MySQL clients. No practical effect.
+
 * `default_order_by_limit`
 
   Used to control the default number of items returned after OrderBy. The default value is -1, and the maximum number of records after the query is returned by default, and the upper limit is the MAX_VALUE of the long data type.

docs/zh-CN/docs/advanced/variables.md

@@ -175,6 +175,10 @@ SELECT /*+ SET_VAR(query_timeout = 1, enable_partition_cache=true) */ sleep(3);
 
   用于兼容 MySQL 客户端。无实际作用。
 
+- `have_query_cache`
+
+  用于兼容 MySQL 客户端。无实际作用。
+
 - `default_order_by_limit`
 
   用于控制 OrderBy 以后返回的默认条数。默认值为 -1，默认返回查询后的最大条数，上限为 long 数据类型的 MAX_VALUE 值。

fe/fe-core/src/main/java/org/apache/doris/qe/SessionVariable.java

@@ -96,6 +96,7 @@ public class SessionVariable implements Serializable, Writable {
     public static final String SQL_SAFE_UPDATES = "sql_safe_updates";
     public static final String NET_BUFFER_LENGTH = "net_buffer_length";
     public static final String CODEGEN_LEVEL = "codegen_level";
+    public static final String HAVE_QUERY_CACHE =  "have_query_cache";
     // mem limit can't smaller than bufferpool's default page size
     public static final int MIN_EXEC_MEM_LIMIT = 2097152;
     public static final String BATCH_SIZE = "batch_size";
@@ -520,6 +521,9 @@ public class SessionVariable implements Serializable, Writable {
     @VariableMgr.VarAttr(name = CODEGEN_LEVEL)
     public int codegenLevel = 0;
 
+    @VariableMgr.VarAttr(name = HAVE_QUERY_CACHE, flag = VariableMgr.READ_ONLY)
+    public boolean haveQueryCache = false;
+
     // 4096 minus 16 + 16 bytes padding that in padding pod array
     @VariableMgr.VarAttr(name = BATCH_SIZE, fuzzy = true)
     public int batchSize = 4064;
@@ -1344,6 +1348,10 @@ public class SessionVariable implements Serializable, Writable {
         return codegenLevel;
     }
 
+    public boolean getHaveQueryCache() {
+        return haveQueryCache;
+    }
+
     /**
      * setMaxExecMemByte.
      **/