Refactor some code.

src/Core/ColumnWithTypeAndName.cpp

@@ -28,11 +28,8 @@ bool ColumnWithTypeAndName::operator==(const ColumnWithTypeAndName & other) cons
 }
 
 
-void ColumnWithTypeAndName::dumpStructure(WriteBuffer & out, bool escaped) const
+void ColumnWithTypeAndName::dumpStructure(WriteBuffer & out) const
 {
-    if (escaped)
-        writeEscapedString(name, out);
-    else
     out << name;
 
     if (type)

src/Core/ColumnWithTypeAndName.h

@@ -33,7 +33,7 @@ struct ColumnWithTypeAndName
     ColumnWithTypeAndName cloneEmpty() const;
     bool operator==(const ColumnWithTypeAndName & other) const;
 
-    void dumpStructure(WriteBuffer & out, bool escaped = false) const;
+    void dumpStructure(WriteBuffer & out) const;
     String dumpStructure() const;
 };
 #pragma GCC diagnostic pop

src/Core/SortDescription.cpp

@@ -1,40 +1,40 @@
 #include <Core/SortDescription.h>
 #include <Core/Block.h>
+#include <IO/Operators.h>
 
 namespace DB
 {
 
-String dumpSortDescription(const SortDescription & description, const Block & header)
+void dumpSortDescription(const SortDescription & description, const Block & header, WriteBuffer & out)
 {
-    String res;
+    bool first = true;
 
     for (const auto & desc : description)
     {
-        if (!res.empty())
+        if (!first)
-            res += ", ";
+            out << ", ";
+        first = false;
 
         if (!desc.column_name.empty())
-            res += desc.column_name;
+            out << desc.column_name;
         else
         {
             if (desc.column_number < header.columns())
-                res += header.getByPosition(desc.column_number).name;
+                out << header.getByPosition(desc.column_number).name;
             else
-                res += "?";
+                out << "?";
 
-            res += " (pos " + std::to_string(desc.column_number) + ")";
+            out << " (pos " << desc.column_number << ")";
         }
 
         if (desc.direction > 0)
-            res += " ASC";
+            out << " ASC";
         else
-            res += " DESC";
+            out << " DESC";
 
         if (desc.with_fill)
-            res += " WITH FILL";
+            out << " WITH FILL";
     }
-
-    return res;
 }
 
 }

src/Core/SortDescription.h

@@ -72,6 +72,6 @@ struct SortColumnDescription
 using SortDescription = std::vector<SortColumnDescription>;
 
 class Block;
-String dumpSortDescription(const SortDescription & description, const Block & header);
+void dumpSortDescription(const SortDescription & description, const Block & header, WriteBuffer & out);
 
 }

src/Interpreters/AggregateDescription.cpp

@@ -1,82 +1,101 @@
 #include <Interpreters/AggregateDescription.h>
 #include <Common/FieldVisitors.h>
+#include <IO/Operators.h>
 
 namespace DB
 {
 
-Strings AggregateDescription::explain() const
+void AggregateDescription::explain(WriteBuffer & out, size_t ident) const
 {
-    Strings res;
+    String prefix(ident, ' ');
 
-    res.emplace_back(column_name);
+    out << prefix << column_name << '\n';
 
-    auto get_params_string = [](const Array & arr)
+    auto dump_params = [&](const Array & arr)
     {
-        String params_str;
+        bool first = true;
         for (const auto & param : arr)
         {
-            if (!params_str.empty())
+            if (!first)
-                params_str += ", ";
+                out << ", ";
 
-            params_str += applyVisitor(FieldVisitorToString(), param);
+            first = false;
+
+            out << applyVisitor(FieldVisitorToString(), param);
         }
-
-        return params_str;
     };
 
     if (function)
     {
-        String types_str;
+        out << prefix << "  Function: " << function->getName();
+
+        const auto & params = function->getParameters();
+        if (!params.empty())
+        {
+            out << "(";
+            dump_params(params);
+            out << ")";
+        }
+
+        out << "(";
+
+        bool first = true;
         for (const auto & type : function->getArgumentTypes())
         {
-            if (!types_str.empty())
+            if (!first)
-                types_str += ", ";
+                out << ", ";
+            first = false;
 
-            types_str += type->getName();
+            out << type->getName();
         }
 
-        auto params_str = get_params_string(function->getParameters());
+        out << ")\n";
-        if (!params_str.empty())
-            params_str = "(" + params_str + ")";
-
-        res.emplace_back("  Function: " + function->getName() + params_str + '(' + types_str + ") -> " +
-                         function->getReturnType()->getName());
     }
     else
-        res.emplace_back("  Function: nullptr");
+        out << prefix << "  Function: nullptr\n";
 
     if (!parameters.empty())
-        res.emplace_back("  Parameters: " + get_params_string(parameters));
+    {
+        out << prefix << "  Parameters: ";
+        dump_params(parameters);
+        out << '\n';
+    }
 
-    String arguments_names_str;
+    out << prefix << "  Arguments: ";
+
+    if (argument_names.empty())
+        out << "none\n";
+    else
+    {
+        bool first = true;
         for (const auto & arg : argument_names)
         {
-        if (!arguments_names_str.empty())
+            if (!first)
-            arguments_names_str += ", ";
+                out << ", ";
+            first = false;
 
-        arguments_names_str += arg;
+            out << arg;
+        }
+        out << "\n";
     }
 
-    if (arguments_names_str.empty())
+    out << prefix << "  Argument positions: ";
-        arguments_names_str = "none";
 
-    res.emplace_back("  Arguments: " + arguments_names_str);
+    if (arguments.empty())
-
+        out << "none\n";
-    String arguments_pos_str;
+    else
+    {
+        bool first = true;
         for (auto arg : arguments)
         {
-        if (!arguments_pos_str.empty())
+            if (!first)
-            arguments_pos_str += ", ";
+                out << ", ";
+            first = false;
 
-        arguments_pos_str += std::to_string(arg);
+            out << arg;
+        }
+        out << '\n';
     }
-
-    if (arguments_pos_str.empty())
-        arguments_pos_str = "none";
-
-    res.emplace_back("  Argument positions: " + arguments_pos_str);
-
-    return res;
 }
 
 }

src/Interpreters/AggregateDescription.h

@@ -16,7 +16,7 @@ struct AggregateDescription
     Names argument_names;    /// used if no `arguments` are specified.
     String column_name;      /// What name to use for a column with aggregate function values
 
-    Strings explain() const; /// Get description for EXPLAIN query.
+    void explain(WriteBuffer & out, size_t ident) const; /// Get description for EXPLAIN query.
 };
 
 using AggregateDescriptions = std::vector<AggregateDescription>;

src/Interpreters/Aggregator.cpp

@@ -29,6 +29,7 @@
 #include <AggregateFunctions/AggregateFunctionArray.h>
 #include <AggregateFunctions/AggregateFunctionState.h>
 #include <Disks/StoragePolicy.h>
+#include <IO/Operators.h>
 
 
 namespace ProfileEvents
@@ -150,45 +151,42 @@ Block Aggregator::Params::getHeader(
     return materializeBlock(res);
 }
 
-Strings Aggregator::Params::explain() const
+void Aggregator::Params::explain(WriteBuffer & out, size_t ident) const
 {
     Strings res;
     const auto & header = src_header ? src_header
                                      : intermediate_header;
-    String keys_str;
+
+    String prefix(ident, ' ');
+
+    {
+        /// Dump keys.
+        out << prefix << "Keys: ";
+
+        bool first = true;
         for (auto key : keys)
         {
-        if (!keys_str.empty())
+            if (!first)
-            keys_str += ", ";
+                out << ", ";
+            first = false;
 
             if (key >= header.columns())
-            keys_str += "unknown position " + std::to_string(key);
+                out << "unknown position " << key;
             else
-            keys_str += header.getByPosition(key).name;
+                out << header.getByPosition(key).name;
         }
 
-    res.emplace_back("Keys: " + std::move(keys_str));
+        out << '\n';
+    }
 
     if (!aggregates.empty())
     {
-        bool first = true;
+        out << prefix << "Aggregates:\n";
+
         for (const auto & aggregate : aggregates)
-        {
+            aggregate.explain(out, ident + 4);
-            auto aggregate_strings = aggregate.explain();
-            for (const auto & aggregate_str : aggregate_strings)
-            {
-                if (first)
-                    res.emplace_back("Aggregates: " + aggregate_str);
-                else
-                    res.emplace_back("            " + aggregate_str);
-
-                first = false;
     }
 }
-    }
-
-    return res;
-}
 
 Aggregator::Aggregator(const Params & params_)
     : params(params_),

src/Interpreters/Aggregator.h

@@ -925,7 +925,7 @@ public:
         }
 
         /// Returns keys and aggregated for EXPLAIN query
-        Strings explain() const;
+        void explain(WriteBuffer & out, size_t ident) const;
     };
 
     Aggregator(const Params & params_);

src/Processors/QueryPlan/AggregatingStep.cpp

@@ -152,9 +152,9 @@ void AggregatingStep::transformPipeline(QueryPipeline & pipeline)
     pipeline.enableQuotaForCurrentStreams();
 }
 
-Strings AggregatingStep::describeActions() const
+void AggregatingStep::describeActions(FormatSettings & settings) const
 {
-    return params.explain();
+    params.explain(settings.out, settings.offset);
 }
 
 void AggregatingStep::describePipeline(FormatSettings & settings) const

src/Processors/QueryPlan/AggregatingStep.h

@@ -28,7 +28,7 @@ public:
 
     void transformPipeline(QueryPipeline & pipeline) override;
 
-    Strings describeActions() const override;
+    void describeActions(FormatSettings &) const override;
     void describePipeline(FormatSettings & settings) const override;
 
 private:

src/Processors/QueryPlan/ConvertingStep.cpp

@@ -1,6 +1,7 @@
 #include <Processors/QueryPlan/ConvertingStep.h>
 #include <Processors/QueryPipeline.h>
 #include <Processors/Transforms/ConvertingTransform.h>
+#include <IO/Operators.h>
 
 namespace DB
 {
@@ -46,19 +47,19 @@ void ConvertingStep::transformPipeline(QueryPipeline & pipeline)
     });
 }
 
-Strings ConvertingStep::describeActions() const
+void ConvertingStep::describeActions(FormatSettings & settings) const
 {
     const auto & header = input_streams[0].header;
     auto conversion = ConvertingTransform(header, result_header, ConvertingTransform::MatchColumnsMode::Name)
             .getConversion();
 
-    Strings res;
+    auto dump_description = [&](const ColumnWithTypeAndName & elem, bool is_const)
-
-    auto get_description = [](const ColumnWithTypeAndName & elem, bool is_const)
     {
-        return elem.name + " " + elem.type->getName() + (is_const ? " Const" : "");
+        settings.out << elem.name << ' ' << elem.type->getName() << (is_const ? " Const" : "") << '\n';
     };
 
+    String prefix(settings.offset, ' ');
+
     for (size_t i = 0; i < conversion.size(); ++i)
     {
         const auto & from = header.getByPosition(conversion[i]);
@@ -67,13 +68,19 @@ Strings ConvertingStep::describeActions() const
         bool from_const = from.column && isColumnConst(*from.column);
         bool to_const = to.column && isColumnConst(*to.column);
 
+        settings.out << prefix;
+
         if (from.name == to.name && from.type->equals(*to.type) && from_const == to_const)
-            res.emplace_back(get_description(from, from_const));
+            dump_description(from, from_const);
         else
-            res.emplace_back(get_description(to, to_const) + " <- " + get_description(from, from_const));
+        {
+            dump_description(to, to_const);
+            settings.out << " <- ";
+            dump_description(from, from_const);
         }
 
-    return res;
+        settings.out << '\n';
+    }
 }
 
 }

src/Processors/QueryPlan/ConvertingStep.h

@@ -13,7 +13,7 @@ public:
 
     void transformPipeline(QueryPipeline & pipeline) override;
 
-    Strings describeActions() const override;
+    void describeActions(FormatSettings & settings) const override;
 
 private:
     Block result_header;

src/Processors/QueryPlan/CreatingSetsStep.cpp

@@ -1,6 +1,7 @@
 #include <Processors/QueryPlan/CreatingSetsStep.h>
 #include <Processors/QueryPipeline.h>
 #include <Processors/Transforms/CreatingSetsTransform.h>
+#include <IO/Operators.h>
 
 namespace DB
 {
@@ -35,21 +36,20 @@ void CreatingSetsStep::transformPipeline(QueryPipeline & pipeline)
     pipeline.addCreatingSetsTransform(std::move(creating_sets));
 }
 
-Strings CreatingSetsStep::describeActions() const
+void CreatingSetsStep::describeActions(FormatSettings & settings) const
 {
-    Strings res;
+    String prefix(settings.offset, ' ');
+
     for (const auto & set : subqueries_for_sets)
     {
-        String str;
+        settings.out << prefix;
         if (set.second.set)
-            str += "Set: ";
+            settings.out << "Set: ";
         else if (set.second.join)
-            str += "Join: ";
+            settings.out << "Join: ";
 
-        str += set.first;
+        settings.out << set.first << '\n';
     }
-
-    return res;
 }
 
 }

src/Processors/QueryPlan/CreatingSetsStep.h

@@ -19,7 +19,7 @@ public:
 
     void transformPipeline(QueryPipeline & pipeline) override;
 
-    Strings describeActions() const override;
+    void describeActions(FormatSettings & settings) const override;
 
 private:
     SubqueriesForSets subqueries_for_sets;

src/Processors/QueryPlan/DistinctStep.cpp

@@ -1,6 +1,7 @@
 #include <Processors/QueryPlan/DistinctStep.h>
 #include <Processors/Transforms/DistinctTransform.h>
 #include <Processors/QueryPipeline.h>
+#include <IO/Operators.h>
 
 namespace DB
 {
@@ -65,18 +66,27 @@ void DistinctStep::transformPipeline(QueryPipeline & pipeline)
     });
 }
 
-Strings DistinctStep::describeActions() const
+void DistinctStep::describeActions(FormatSettings & settings) const
 {
-    String res;
+    String prefix(settings.offset, ' ');
+    settings.out << prefix << "Columns: ";
+
+    if (columns.empty())
+        settings.out << "none";
+    else
+    {
+        bool first = true;
         for (const auto & column : columns)
         {
-        if (!res.empty())
+            if (!first)
-            res += ", ";
+                settings.out << ", ";
+            first = false;
 
-        res += column;
+            settings.out << column;
+        }
     }
 
-    return {"Columns: " + res};
+    settings.out << '\n';
 }
 
 }

src/Processors/QueryPlan/DistinctStep.h

@@ -19,7 +19,7 @@ public:
 
     void transformPipeline(QueryPipeline & pipeline) override;
 
-    Strings describeActions() const override;
+    void describeActions(FormatSettings & settings) const override;
 
 private:
     SizeLimits set_size_limits;

src/Processors/QueryPlan/ExpressionStep.cpp

@@ -3,6 +3,7 @@
 #include <Processors/QueryPipeline.h>
 #include <Processors/Transforms/InflatingExpressionTransform.h>
 #include <Interpreters/ExpressionActions.h>
+#include <IO/Operators.h>
 
 namespace DB
 {
@@ -50,19 +51,23 @@ void ExpressionStep::transformPipeline(QueryPipeline & pipeline)
     });
 }
 
-static Strings getActionsDescription(const ExpressionActionsPtr & expression)
+static void doDescribeActions(const ExpressionActionsPtr & expression, IQueryPlanStep::FormatSettings & settings)
 {
-    Strings res;
+    String prefix(settings.offset, ' ');
-    for (const auto & action : expression->getActions())
+    bool first = true;
-        res.emplace_back((res.empty() ? "Actions: "
-                                      : "         ") + action.toString());
 
-    return res;
+    for (const auto & action : expression->getActions())
+    {
+        settings.out << prefix << (first ? "Actions: "
+                                         : "         ");
+        first = false;
+        settings.out << action.toString() << '\n';
+    }
 }
 
-Strings ExpressionStep::describeActions() const
+void ExpressionStep::describeActions(FormatSettings & settings) const
 {
-    return getActionsDescription(expression);
+    doDescribeActions(expression, settings);
 }
 
 InflatingExpressionStep::InflatingExpressionStep(const DataStream & input_stream_, ExpressionActionsPtr expression_)
@@ -93,9 +98,9 @@ void InflatingExpressionStep::transformPipeline(QueryPipeline & pipeline)
     });
 }
 
-Strings InflatingExpressionStep::describeActions() const
+void InflatingExpressionStep::describeActions(FormatSettings & settings) const
 {
-    return getActionsDescription(expression);
+    doDescribeActions(expression, settings);
 }
 
 }

src/Processors/QueryPlan/ExpressionStep.h

@@ -20,7 +20,7 @@ public:
 
     void transformPipeline(QueryPipeline & pipeline) override;
 
-    Strings describeActions() const override;
+    void describeActions(FormatSettings & settings) const override;
 
 private:
     ExpressionActionsPtr expression;
@@ -37,7 +37,7 @@ public:
 
     void transformPipeline(QueryPipeline & pipeline) override;
 
-    Strings describeActions() const override;
+    void describeActions(FormatSettings & settings) const override;
 
 private:
     ExpressionActionsPtr expression;

src/Processors/QueryPlan/FillingStep.cpp

@@ -1,6 +1,7 @@
 #include <Processors/QueryPlan/FillingStep.h>
 #include <Processors/Transforms/FillingTransform.h>
 #include <Processors/QueryPipeline.h>
+#include <IO/Operators.h>
 
 namespace DB
 {
@@ -27,9 +28,11 @@ void FillingStep::transformPipeline(QueryPipeline & pipeline)
     });
 }
 
-Strings FillingStep::describeActions() const
+void FillingStep::describeActions(FormatSettings & settings) const
 {
-    return {"Sort description: " + dumpSortDescription(sort_description, input_streams.front().header)};
+    settings.out << String(settings.offset, ' ');
+    dumpSortDescription(sort_description, input_streams.front().header, settings.out);
+    settings.out << '\n';
 }
 
 }

src/Processors/QueryPlan/FillingStep.h

@@ -14,7 +14,7 @@ public:
 
     void transformPipeline(QueryPipeline & pipeline) override;
 
-    Strings describeActions() const override;
+    void describeActions(FormatSettings & settings) const override;
 
 private:
     SortDescription sort_description;

src/Processors/QueryPlan/FilterStep.cpp

@@ -2,6 +2,7 @@
 #include <Processors/Transforms/FilterTransform.h>
 #include <Processors/QueryPipeline.h>
 #include <Interpreters/ExpressionActions.h>
+#include <IO/Operators.h>
 
 namespace DB
 {
@@ -55,16 +56,19 @@ void FilterStep::transformPipeline(QueryPipeline & pipeline)
     });
 }
 
-Strings FilterStep::describeActions() const
+void FilterStep::describeActions(FormatSettings & settings) const
 {
-    Strings res;
+    String prefix(settings.offset, ' ');
-    res.emplace_back("Filter column: " + filter_column_name);
+    settings.out << prefix << "Filter column: " << filter_column_name << '\n';
 
+    bool first = true;
     for (const auto & action : expression->getActions())
-        res.emplace_back((res.size() == 1 ? "Actions: "
+    {
-                                          : "         ") + action.toString());
+        settings.out << prefix << (first ? "Actions: "
-
+                                         : "         ");
-    return res;
+        first = false;
+        settings.out << action.toString() << '\n';
+    }
 }
 
 }

src/Processors/QueryPlan/FilterStep.h

@@ -19,7 +19,7 @@ public:
     String getName() const override { return "Filter"; }
     void transformPipeline(QueryPipeline & pipeline) override;
 
-    Strings describeActions() const override;
+    void describeActions(FormatSettings & settings) const override;
 
 private:
     ExpressionActionsPtr expression;

src/Processors/QueryPlan/FinishSortingStep.cpp

@@ -4,6 +4,7 @@
 #include <Processors/Merges/MergingSortedTransform.h>
 #include <Processors/Transforms/PartialSortingTransform.h>
 #include <Processors/Transforms/FinishSortingTransform.h>
+#include <IO/Operators.h>
 
 namespace DB
 {
@@ -69,17 +70,20 @@ void FinishSortingStep::transformPipeline(QueryPipeline & pipeline)
     }
 }
 
-Strings FinishSortingStep::describeActions() const
+void FinishSortingStep::describeActions(FormatSettings & settings) const
 {
-    Strings res = {
+    String prefix(settings.offset, ' ');
-        "Prefix sort description: " + dumpSortDescription(prefix_description, input_streams.front().header),
+
-        "Result sort description: " + dumpSortDescription(result_description, input_streams.front().header)
+    settings.out << prefix << "Prefix sort description: ";
-    };
+    dumpSortDescription(prefix_description, input_streams.front().header, settings.out);
+    settings.out << '\n';
+
+    settings.out << prefix << "Result sort description: ";
+    dumpSortDescription(result_description, input_streams.front().header, settings.out);
+    settings.out << '\n';
 
     if (limit)
-        res.emplace_back("Limit " + std::to_string(limit));
+        settings.out << prefix << "Limit " << limit << '\n';
-
-    return res;
 }
 
 }

src/Processors/QueryPlan/FinishSortingStep.h

@@ -19,7 +19,7 @@ public:
 
     void transformPipeline(QueryPipeline & pipeline) override;
 
-    Strings describeActions() const override;
+    void describeActions(FormatSettings & settings) const override;
 
 private:
     SortDescription prefix_description;

src/Processors/QueryPlan/IQueryPlanStep.cpp

@@ -45,7 +45,7 @@ static void doDescribeHeader(const Block & header, size_t count, IQueryPlanStep:
             settings.out << prefix;
 
         first = false;
-        elem.dumpStructure(settings.out, true);
+        elem.dumpStructure(settings.out);
         settings.out << '\n';
     }
 }

src/Processors/QueryPlan/IQueryPlanStep.h

@@ -65,7 +65,7 @@ public:
     };
 
     /// Get detailed description of step actions. This is shown in EXPLAIN query with options `actions = 1`.
-    virtual Strings describeActions() const { return {}; }
+    virtual void describeActions(FormatSettings & /*settings*/) const {}
 
     /// Get description of processors added in current step. Should be called after updatePipeline().
     virtual void describePipeline(FormatSettings & /*settings*/) const {}

src/Processors/QueryPlan/LimitByStep.cpp

@@ -1,6 +1,7 @@
 #include <Processors/QueryPlan/LimitByStep.h>
 #include <Processors/Transforms/LimitByTransform.h>
 #include <Processors/QueryPipeline.h>
+#include <IO/Operators.h>
 
 namespace DB
 {
@@ -37,23 +38,30 @@ void LimitByStep::transformPipeline(QueryPipeline & pipeline)
     });
 }
 
-Strings LimitByStep::describeActions() const
+void LimitByStep::describeActions(FormatSettings & settings) const
 {
-    Strings res;
+    String prefix(settings.offset, ' ');
-    String columns_str;
+
+    settings.out << prefix << "Columns: ";
+
+    if (columns.empty())
+        settings.out << "none\n";
+    else
+    {
+        bool first = true;
         for (const auto & column : columns)
         {
-        if (!columns_str.empty())
+            if (!first)
-            columns_str += ", ";
+                settings.out << ", ";
+            first = false;
 
-        columns_str += column;
+            settings.out << column;
+        }
+        settings.out << '\n';
     }
 
-    return {
+    settings.out << prefix << "Length " << group_length << '\n';
-        "Columns: " + columns_str,
+    settings.out << prefix << "Offset " << group_offset << '\n';
-        "Length " + std::to_string(group_length),
-        "Offset " + std::to_string(group_offset),
-    };
 }
 
 }

src/Processors/QueryPlan/LimitByStep.h

@@ -15,7 +15,7 @@ public:
 
     void transformPipeline(QueryPipeline & pipeline) override;
 
-    Strings describeActions() const override;
+    void describeActions(FormatSettings & settings) const override;
 
 private:
     size_t group_length;
@@ -24,5 +24,3 @@ private:
 };
 
 }
-
-

src/Processors/QueryPlan/LimitStep.cpp

@@ -1,6 +1,7 @@
 #include <Processors/QueryPlan/LimitStep.h>
 #include <Processors/QueryPipeline.h>
 #include <Processors/LimitTransform.h>
+#include <IO/Operators.h>
 
 namespace DB
 {
@@ -35,30 +36,30 @@ void LimitStep::transformPipeline(QueryPipeline & pipeline)
     pipeline.addPipe({std::move(transform)});
 }
 
-Strings LimitStep::describeActions() const
+void LimitStep::describeActions(FormatSettings & settings) const
 {
-    Strings res;
+    String prefix(settings.offset, ' ');
-    res.emplace_back("Limit " + std::to_string(limit));
+    settings.out << prefix << "Limit " << limit << '\n';
-    res.emplace_back("Offset " + std::to_string(offset));
+    settings.out << prefix << "Offset " << offset << '\n';
 
     if (with_ties || always_read_till_end)
     {
+        settings.out << prefix;
+
         String str;
         if (with_ties)
-            str += "WITH TIES";
+            settings.out << "WITH TIES";
 
         if (always_read_till_end)
         {
-            if (!str.empty())
+            if (!with_ties)
-                str += ", ";
+                settings.out << ", ";
 
-            str += "Reads all data";
+            settings.out << "Reads all data";
         }
 
-        res.emplace_back(str);
+        settings.out << '\n';
     }
-
-    return res;
 }
 
 }

src/Processors/QueryPlan/LimitStep.h

@@ -20,7 +20,7 @@ public:
 
     void transformPipeline(QueryPipeline & pipeline) override;
 
-    Strings describeActions() const override;
+    void describeActions(FormatSettings & settings) const override;
 
 private:
     size_t limit;

src/Processors/QueryPlan/MergeSortingStep.cpp

@@ -1,6 +1,7 @@
 #include <Processors/QueryPlan/MergeSortingStep.h>
 #include <Processors/QueryPipeline.h>
 #include <Processors/Transforms/MergeSortingTransform.h>
+#include <IO/Operators.h>
 
 namespace DB
 {
@@ -48,15 +49,15 @@ void MergeSortingStep::transformPipeline(QueryPipeline & pipeline)
     });
 }
 
-Strings MergeSortingStep::describeActions() const
+void MergeSortingStep::describeActions(FormatSettings & settings) const
 {
-    Strings res = {"Sort description: " + dumpSortDescription(description, input_streams.front().header)};
+    String prefix(settings.offset, ' ');
+    settings.out << prefix << "Sort description: ";
+    dumpSortDescription(description, input_streams.front().header, settings.out);
+    settings.out << '\n';
 
     if (limit)
-        res.emplace_back("Limit " + std::to_string(limit));
+        settings.out << prefix << "Limit " << limit << '\n';
-
-    return res;
 }
 
-
 }

src/Processors/QueryPlan/MergeSortingStep.h

@@ -24,7 +24,7 @@ public:
 
     void transformPipeline(QueryPipeline & pipeline) override;
 
-    Strings describeActions() const override;
+    void describeActions(FormatSettings & settings) const override;
 
 private:
     SortDescription description;

src/Processors/QueryPlan/MergingAggregatedStep.cpp

@@ -63,9 +63,9 @@ void MergingAggregatedStep::transformPipeline(QueryPipeline & pipeline)
     pipeline.enableQuotaForCurrentStreams();
 }
 
-Strings MergingAggregatedStep::describeActions() const
+void MergingAggregatedStep::describeActions(FormatSettings & settings) const
 {
-    return params->params.explain();
+    return params->params.explain(settings.out, settings.offset);
 }
 
 }

src/Processors/QueryPlan/MergingAggregatedStep.h

@@ -22,7 +22,7 @@ public:
 
     void transformPipeline(QueryPipeline & pipeline) override;
 
-    Strings describeActions() const override;
+    void describeActions(FormatSettings & settings) const override;
 
 private:
     AggregatingTransformParamsPtr params;

src/Processors/QueryPlan/MergingSortedStep.cpp

@@ -1,6 +1,7 @@
 #include <Processors/QueryPlan/MergingSortedStep.h>
 #include <Processors/QueryPipeline.h>
 #include <Processors/Merges/MergingSortedTransform.h>
+#include <IO/Operators.h>
 
 namespace DB
 {
@@ -46,14 +47,12 @@ void MergingSortedStep::transformPipeline(QueryPipeline & pipeline)
     }
 }
 
-Strings MergingSortedStep::describeActions() const
+void MergingSortedStep::describeActions(FormatSettings & settings) const
 {
-    Strings res = {"Sort description: " + dumpSortDescription(sort_description, input_streams.front().header)};
+    String prefix(settings.offset, ' ');
-
+    settings.out << prefix << "Sort description: ";
-    if (limit)
+    dumpSortDescription(sort_description, input_streams.front().header, settings.out);
-        res.emplace_back("Limit " + std::to_string(limit));
+    settings.out << '\n';
-
-    return res;
 }
 
 }

src/Processors/QueryPlan/MergingSortedStep.h

@@ -20,7 +20,7 @@ public:
 
     void transformPipeline(QueryPipeline & pipeline) override;
 
-    Strings describeActions() const override;
+    void describeActions(FormatSettings & settings) const override;
 
 private:
     SortDescription sort_description;

src/Processors/QueryPlan/OffsetStep.cpp

@@ -1,6 +1,7 @@
 #include <Processors/QueryPlan/OffsetStep.h>
 #include <Processors/OffsetTransform.h>
 #include <Processors/QueryPipeline.h>
+#include <IO/Operators.h>
 
 namespace DB
 {
@@ -30,9 +31,9 @@ void OffsetStep::transformPipeline(QueryPipeline & pipeline)
     });
 }
 
-Strings OffsetStep::describeActions() const
+void OffsetStep::describeActions(FormatSettings & settings) const
 {
-    return {"Offset " + std::to_string(offset)};
+    settings.out << String(settings.offset, ' ') << "Offset " << offset << '\n';
 }
 
 }

src/Processors/QueryPlan/OffsetStep.h

@@ -14,7 +14,7 @@ public:
 
     void transformPipeline(QueryPipeline & pipeline) override;
 
-    Strings describeActions() const override;
+    void describeActions(FormatSettings & settings) const override;
 
 private:
     size_t offset;

src/Processors/QueryPlan/PartialSortingStep.cpp

@@ -2,6 +2,7 @@
 #include <Processors/QueryPipeline.h>
 #include <Processors/Transforms/PartialSortingTransform.h>
 #include <Processors/Transforms/LimitsCheckingTransform.h>
+#include <IO/Operators.h>
 
 namespace DB
 {
@@ -50,14 +51,12 @@ void PartialSortingStep::transformPipeline(QueryPipeline & pipeline)
     });
 }
 
-Strings PartialSortingStep::describeActions() const
+void PartialSortingStep::describeActions(FormatSettings & settings) const
 {
-    Strings res = {"Sort description: " + dumpSortDescription(sort_description, input_streams.front().header)};
+    String prefix(settings.offset, ' ');
-
+    settings.out << prefix << "Sort description: ";
-    if (limit)
+    dumpSortDescription(sort_description, input_streams.front().header, settings.out);
-        res.emplace_back("Limit " + std::to_string(limit));
+    settings.out << '\n';
-
-    return res;
 }
 
 }

src/Processors/QueryPlan/PartialSortingStep.h

@@ -19,7 +19,7 @@ public:
 
     void transformPipeline(QueryPipeline & pipeline) override;
 
-    Strings describeActions() const override;
+    void describeActions(FormatSettings & settings) const override;
 
 private:
     SortDescription sort_description;

src/Processors/QueryPlan/QueryPlan.cpp

@@ -178,60 +178,55 @@ void QueryPlan::addInterpreterContext(std::shared_ptr<Context> context)
 
 
 static void explainStep(
-    WriteBuffer & buffer, IQueryPlanStep & step, size_t ident, const QueryPlan::ExplainPlanOptions & options)
+    const IQueryPlanStep & step,
+    IQueryPlanStep::FormatSettings & settings,
+    const QueryPlan::ExplainPlanOptions & options)
 {
-    std::string prefix(ident, ' ');
+    std::string prefix(settings.offset, ' ');
-    buffer << prefix;
+    settings.out << prefix;
-    buffer << step.getName();
+    settings.out << step.getName();
 
     const auto & description = step.getStepDescription();
     if (options.description && !description.empty())
-        buffer <<" (" << description << ')';
+        settings.out <<" (" << description << ')';
 
-    buffer.write('\n');
+    settings.out.write('\n');
 
     if (options.header)
     {
-        buffer << prefix;
+        settings.out << prefix;
 
         if (!step.hasOutputStream())
-            buffer << "No header";
+            settings.out << "No header";
         else if (!step.getOutputStream().header)
-            buffer << "Empty header";
+            settings.out << "Empty header";
         else
         {
-            buffer << "Header: ";
+            settings.out << "Header: ";
             bool first = true;
 
             for (const auto & elem : step.getOutputStream().header)
             {
                 if (!first)
-                    buffer << "\n" << prefix << "        ";
+                    settings.out << "\n" << prefix << "        ";
 
                 first = false;
-                elem.dumpStructure(buffer, true);
+                elem.dumpStructure(settings.out);
             }
         }
 
-        buffer.write('\n');
+        settings.out.write('\n');
     }
 
     if (options.actions)
-    {
+        step.describeActions(settings);
-        auto actions = step.describeActions();
-        if (!actions.empty())
-        {
-            for (auto & action : actions)
-                buffer << prefix << action << '\n';
-        }
-    }
 }
 
 void QueryPlan::explainPlan(WriteBuffer & buffer, const ExplainPlanOptions & options)
 {
     checkInitialized();
 
-    size_t ident = 2;
+    IQueryPlanStep::FormatSettings settings{.out = buffer, .write_header = options.header};
 
     struct Frame
     {
@@ -249,7 +244,8 @@ void QueryPlan::explainPlan(WriteBuffer & buffer, const ExplainPlanOptions & opt
 
         if (!frame.is_description_printed)
         {
-            explainStep(buffer, *frame.node->step, (stack.size() - 1) * ident, options);
+            settings.offset = (stack.size() - 1) * settings.ident;
+            explainStep(*frame.node->step, settings, options);
             frame.is_description_printed = true;
         }
 

src/Processors/QueryPlan/TotalsHavingStep.cpp

@@ -3,6 +3,7 @@
 #include <Processors/QueryPipeline.h>
 #include <Processors/Transforms/TotalsHavingTransform.h>
 #include <Interpreters/ExpressionActions.h>
+#include <IO/Operators.h>
 
 namespace DB
 {
@@ -62,17 +63,20 @@ static String totalsModeToString(TotalsMode totals_mode, double auto_include_thr
     __builtin_unreachable();
 }
 
-Strings TotalsHavingStep::describeActions() const
+void TotalsHavingStep::describeActions(FormatSettings & settings) const
 {
-    Strings res;
+    String prefix(settings.offset, ' ');
-    res.emplace_back("Filter column: " + filter_column_name);
+    settings.out << prefix << "Filter column: " << filter_column_name << '\n';
-    res.emplace_back("Mode: " + totalsModeToString(totals_mode, auto_include_threshold));
+    settings.out << prefix << "Mode: " << totalsModeToString(totals_mode, auto_include_threshold) << '\n';
 
+    bool first = true;
     for (const auto & action : expression->getActions())
-        res.emplace_back((res.size() == 2 ? "Actions: "
+    {
-                                          : "         ") + action.toString());
+        settings.out << prefix << (first ? "Actions: "
-
+                                         : "         ");
-    return res;
+        first = false;
+        settings.out << action.toString() << '\n';
+    }
 }
 
 }

src/Processors/QueryPlan/TotalsHavingStep.h

@@ -25,7 +25,7 @@ public:
 
     void transformPipeline(QueryPipeline & pipeline) override;
 
-    Strings describeActions() const override;
+    void describeActions(FormatSettings & settings) const override;
 
 private:
     bool overflow_row;