Add explicit table info to the scan node of query plan and pipeline

:) explain plan select * from table1 t1 left join table2 t2 on t1.name = t2.name;
┌─explain──────────────────────────────────────────────────────────────────────────────────────┐
│ Expression ((Projection + Before ORDER BY))                                                  │
│   Join (JOIN)                                                                                │
│     Expression (Before JOIN)                                                                 │
│       SettingQuotaAndLimits (Set limits and quota after reading from storage)                │
│         ReadFromMergeTree (default.table1)                                                   │
│     Expression ((Joined actions + (Rename joined columns + (Projection + Before ORDER BY)))) │
│       SettingQuotaAndLimits (Set limits and quota after reading from storage)                │
│         ReadFromMergeTree (default.table2)                                                   │
└──────────────────────────────────────────────────────────────────────────────────────────────┘

:) explain pipeline select * from table1 t1 left join table2 t2 on t1.name = t2.name;
┌─explain──────────────────────────────────────────┐
│ (Expression)                                     │
│ ExpressionTransform × 24                         │
│   (Join)                                         │
│   JoiningTransform × 24 2 → 1                    │
│     Resize 1 → 24                                │
│       FillingRightJoinSide                       │
│         Resize 24 → 1                            │
│           (Expression)                           │
│           ExpressionTransform × 24               │
│             (SettingQuotaAndLimits)              │
│               (ReadFromMergeTree default.table1) │
│               MergeTreeThread × 24 0 → 1         │
│           (Expression)                           │
│           ExpressionTransform × 24               │
│             (SettingQuotaAndLimits)              │
│               (ReadFromMergeTree default.table2) │
│               MergeTreeThread × 24 0 → 1         │
└──────────────────────────────────────────────────┘

src/Processors/QueryPlan/QueryPlan.cpp

@@ -1,5 +1,6 @@
 #include <Processors/QueryPlan/QueryPlan.h>
 #include <Processors/QueryPlan/IQueryPlanStep.h>
+#include <Processors/QueryPlan/ReadFromMergeTree.h>
 #include <QueryPipeline/QueryPipelineBuilder.h>
 #include <IO/WriteBuffer.h>
 #include <IO/Operators.h>
@@ -387,7 +388,15 @@ void QueryPlan::explainPlan(WriteBuffer & buffer, const ExplainPlanOptions & opt
 
 static void explainPipelineStep(IQueryPlanStep & step, IQueryPlanStep::FormatSettings & settings)
 {
-    settings.out << String(settings.offset, settings.indent_char) << "(" << step.getName() << ")\n";
+    // Add explicit description to the scan node of pipeline.
+    if (ReadFromMergeTree::READ_FROM_MERGETREE_NAME == step.getName())
+    {
+        settings.out << String(settings.offset, settings.indent_char) << "(" << step.getName() << " " << step.getStepDescription() << ")\n";
+    }
+    else
+    {
+        settings.out << String(settings.offset, settings.indent_char) << "(" << step.getName() << ")\n";
+    }
     size_t current_offset = settings.offset;
     step.describePipeline(settings);
     if (current_offset == settings.offset)

src/Processors/QueryPlan/ReadFromMergeTree.cpp

@@ -45,6 +45,8 @@ namespace ErrorCodes
     extern const int LOGICAL_ERROR;
 }
 
+const std::string ReadFromMergeTree::READ_FROM_MERGETREE_NAME = "ReadFromMergeTree";
+
 static MergeTreeReaderSettings getMergeTreeReaderSettings(const ContextPtr & context)
 {
     const auto & settings = context->getSettingsRef();
@@ -112,6 +114,9 @@ ReadFromMergeTree::ReadFromMergeTree(
 
     if (enable_parallel_reading)
         read_task_callback = context->getMergeTreeReadTaskCallback();
+
+    /// Add explicit description.
+    setStepDescription(data.getStorageID().getFullNameNotQuoted());
 }
 
 Pipe ReadFromMergeTree::readFromPool(

src/Processors/QueryPlan/ReadFromMergeTree.h

@@ -100,7 +100,8 @@ class ReadFromMergeTree final : public ISourceStep
         bool enable_parallel_reading
     );
 
-    String getName() const override { return "ReadFromMergeTree"; }
+    static const std::string READ_FROM_MERGETREE_NAME;
+    String getName() const override { return READ_FROM_MERGETREE_NAME; }
 
     void initializePipeline(QueryPipelineBuilder & pipeline, const BuildQueryPipelineSettings &) override;