Merge pull request #32836 from stigsb/explain-table-override

Add `EXPLAIN TABLE OVERRIDE` for pre-validating overrides.

docs/en/sql-reference/statements/explain.md

@@ -10,7 +10,12 @@ Shows the execution plan of a statement.
 Syntax:
 
 ```sql
-EXPLAIN [AST | SYNTAX | PLAN | PIPELINE] [setting = value, ...] SELECT ... [FORMAT ...]
+EXPLAIN [AST | SYNTAX | PLAN | PIPELINE | TABLE OVERRIDE] [setting = value, ...]
+    [
+      SELECT ... |
+      tableFunction(...) [COLUMNS (...)] [ORDER BY ...] [PARTITION BY ...] [PRIMARY KEY] [SAMPLE BY ...] [TTL ...]
+    ]
+    [FORMAT ...]
 ```
 
 Example:
@@ -412,4 +417,37 @@ Result:
 └──────────┴───────┴───────┴──────┴───────┘
 ```
 
+### EXPLAIN TABLE OVERRIDE {#explain-table-override}
+
+Shows the result of a table override on a table schema accessed through a table function.
+Also does some validation, throwing an exception if the override would have caused some kind of failure.
+
+**Example**
+
+Assume you have a remote MySQL table like this:
+
+```sql
+CREATE TABLE db.tbl (
+    id INT PRIMARY KEY,
+    created DATETIME DEFAULT now()
+)
+```
+
+```sql
+EXPLAIN TABLE OVERRIDE mysql('127.0.0.1:3306', 'db', 'tbl', 'root', 'clickhouse')
+PARTITION BY toYYYYMM(assumeNotNull(created))
+```
+
+Result:
+
+```text
+┌─explain─────────────────────────────────────────────────┐
+│ PARTITION BY uses columns: `created` Nullable(DateTime) │
+└─────────────────────────────────────────────────────────┘
+```
+
+!!! note "Note"
+    The validation is not complete, so a successfull query does not guarantee that the override would
+    not cause issues.
+
 [Оriginal article](https://clickhouse.com/docs/en/sql-reference/statements/explain/) <!--hide-->

src/Common/ErrorCodes.cpp

@@ -605,6 +605,7 @@
     M(634, MONGODB_ERROR) \
     M(635, CANNOT_POLL) \
     M(636, CANNOT_EXTRACT_TABLE_STRUCTURE) \
+    M(637, INVALID_TABLE_OVERRIDE) \
     \
     M(999, KEEPER_EXCEPTION) \
     M(1000, POCO_EXCEPTION) \

src/Interpreters/InterpreterExplainQuery.cpp

@@ -7,10 +7,12 @@
 #include <Interpreters/InterpreterSelectWithUnionQuery.h>
 #include <Interpreters/InterpreterSelectQuery.h>
 #include <Interpreters/Context.h>
+#include <Interpreters/TableOverrideUtils.h>
 #include <Formats/FormatFactory.h>
 #include <Parsers/DumpASTNode.h>
 #include <Parsers/queryToString.h>
 #include <Parsers/ASTExplainQuery.h>
+#include <Parsers/ASTFunction.h>
 #include <Parsers/ASTSelectQuery.h>
 #include <Parsers/ASTSelectWithUnionQuery.h>
 
@@ -250,117 +252,141 @@ QueryPipeline InterpreterExplainQuery::executeImpl()
 
     WriteBufferFromOwnString buf;
     bool single_line = false;
+    bool insert_buf = true;
 
-    if (ast.getKind() == ASTExplainQuery::ParsedAST)
+    switch (ast.getKind())
     {
-        if (ast.getSettings())
-            throw Exception("Settings are not supported for EXPLAIN AST query.", ErrorCodes::UNKNOWN_SETTING);
-
-        dumpAST(*ast.getExplainedQuery(), buf);
-    }
-    else if (ast.getKind() == ASTExplainQuery::AnalyzedSyntax)
-    {
-        if (ast.getSettings())
-            throw Exception("Settings are not supported for EXPLAIN SYNTAX query.", ErrorCodes::UNKNOWN_SETTING);
-
-        ExplainAnalyzedSyntaxVisitor::Data data(getContext());
-        ExplainAnalyzedSyntaxVisitor(data).visit(query);
-
-        ast.getExplainedQuery()->format(IAST::FormatSettings(buf, false));
-    }
-    else if (ast.getKind() == ASTExplainQuery::QueryPlan)
-    {
-        if (!dynamic_cast<const ASTSelectWithUnionQuery *>(ast.getExplainedQuery().get()))
-            throw Exception("Only SELECT is supported for EXPLAIN query", ErrorCodes::INCORRECT_QUERY);
-
-        auto settings = checkAndGetSettings<QueryPlanSettings>(ast.getSettings());
-        QueryPlan plan;
-
-        InterpreterSelectWithUnionQuery interpreter(ast.getExplainedQuery(), getContext(), SelectQueryOptions());
-        interpreter.buildQueryPlan(plan);
-
-        if (settings.optimize)
-            plan.optimize(QueryPlanOptimizationSettings::fromContext(getContext()));
-
-        if (settings.json)
+        case ASTExplainQuery::ParsedAST:
         {
-            /// Add extra layers to make plan look more like from postgres.
-            auto plan_map = std::make_unique<JSONBuilder::JSONMap>();
-            plan_map->add("Plan", plan.explainPlan(settings.query_plan_options));
-            auto plan_array = std::make_unique<JSONBuilder::JSONArray>();
-            plan_array->add(std::move(plan_map));
+            if (ast.getSettings())
+                throw Exception("Settings are not supported for EXPLAIN AST query.", ErrorCodes::UNKNOWN_SETTING);
 
-            auto format_settings = getFormatSettings(getContext());
-            format_settings.json.quote_64bit_integers = false;
-
-            JSONBuilder::FormatSettings json_format_settings{.settings = format_settings};
-            JSONBuilder::FormatContext format_context{.out = buf};
-
-            plan_array->format(json_format_settings, format_context);
-
-            single_line = true;
+            dumpAST(*ast.getExplainedQuery(), buf);
+            break;
         }
-        else
-            plan.explainPlan(buf, settings.query_plan_options);
-    }
-    else if (ast.getKind() == ASTExplainQuery::QueryPipeline)
-    {
-        if (dynamic_cast<const ASTSelectWithUnionQuery *>(ast.getExplainedQuery().get()))
+        case ASTExplainQuery::AnalyzedSyntax:
         {
-            auto settings = checkAndGetSettings<QueryPipelineSettings>(ast.getSettings());
+            if (ast.getSettings())
+                throw Exception("Settings are not supported for EXPLAIN SYNTAX query.", ErrorCodes::UNKNOWN_SETTING);
+
+            ExplainAnalyzedSyntaxVisitor::Data data(getContext());
+            ExplainAnalyzedSyntaxVisitor(data).visit(query);
+
+            ast.getExplainedQuery()->format(IAST::FormatSettings(buf, false));
+            break;
+        }
+        case ASTExplainQuery::QueryPlan:
+        {
+            if (!dynamic_cast<const ASTSelectWithUnionQuery *>(ast.getExplainedQuery().get()))
+                throw Exception("Only SELECT is supported for EXPLAIN query", ErrorCodes::INCORRECT_QUERY);
+
+            auto settings = checkAndGetSettings<QueryPlanSettings>(ast.getSettings());
             QueryPlan plan;
 
             InterpreterSelectWithUnionQuery interpreter(ast.getExplainedQuery(), getContext(), SelectQueryOptions());
             interpreter.buildQueryPlan(plan);
-            auto pipeline = plan.buildQueryPipeline(
+
+            if (settings.optimize)
+                plan.optimize(QueryPlanOptimizationSettings::fromContext(getContext()));
+
+            if (settings.json)
+            {
+                /// Add extra layers to make plan look more like from postgres.
+                auto plan_map = std::make_unique<JSONBuilder::JSONMap>();
+                plan_map->add("Plan", plan.explainPlan(settings.query_plan_options));
+                auto plan_array = std::make_unique<JSONBuilder::JSONArray>();
+                plan_array->add(std::move(plan_map));
+
+                auto format_settings = getFormatSettings(getContext());
+                format_settings.json.quote_64bit_integers = false;
+
+                JSONBuilder::FormatSettings json_format_settings{.settings = format_settings};
+                JSONBuilder::FormatContext format_context{.out = buf};
+
+                plan_array->format(json_format_settings, format_context);
+
+                single_line = true;
+            }
+            else
+                plan.explainPlan(buf, settings.query_plan_options);
+            break;
+        }
+        case ASTExplainQuery::QueryPipeline:
+        {
+            if (dynamic_cast<const ASTSelectWithUnionQuery *>(ast.getExplainedQuery().get()))
+            {
+                auto settings = checkAndGetSettings<QueryPipelineSettings>(ast.getSettings());
+                QueryPlan plan;
+
+                InterpreterSelectWithUnionQuery interpreter(ast.getExplainedQuery(), getContext(), SelectQueryOptions());
+                interpreter.buildQueryPlan(plan);
+                auto pipeline = plan.buildQueryPipeline(
+                    QueryPlanOptimizationSettings::fromContext(getContext()),
+                    BuildQueryPipelineSettings::fromContext(getContext()));
+
+                if (settings.graph)
+                {
+                    /// Pipe holds QueryPlan, should not go out-of-scope
+                    auto pipe = QueryPipelineBuilder::getPipe(std::move(*pipeline));
+                    const auto & processors = pipe.getProcessors();
+
+                    if (settings.compact)
+                        printPipelineCompact(processors, buf, settings.query_pipeline_options.header);
+                    else
+                        printPipeline(processors, buf);
+                }
+                else
+                {
+                    plan.explainPipeline(buf, settings.query_pipeline_options);
+                }
+            }
+            else if (dynamic_cast<const ASTInsertQuery *>(ast.getExplainedQuery().get()))
+            {
+                InterpreterInsertQuery insert(ast.getExplainedQuery(), getContext());
+                auto io = insert.execute();
+                printPipeline(io.pipeline.getProcessors(), buf);
+            }
+            else
+                throw Exception("Only SELECT and INSERT is supported for EXPLAIN PIPELINE query", ErrorCodes::INCORRECT_QUERY);
+            break;
+        }
+        case ASTExplainQuery::QueryEstimates:
+        {
+            if (!dynamic_cast<const ASTSelectWithUnionQuery *>(ast.getExplainedQuery().get()))
+                throw Exception("Only SELECT is supported for EXPLAIN ESTIMATE query", ErrorCodes::INCORRECT_QUERY);
+
+            auto settings = checkAndGetSettings<QueryPlanSettings>(ast.getSettings());
+            QueryPlan plan;
+
+            InterpreterSelectWithUnionQuery interpreter(ast.getExplainedQuery(), getContext(), SelectQueryOptions());
+            interpreter.buildQueryPlan(plan);
+            // collect the selected marks, rows, parts during build query pipeline.
+            plan.buildQueryPipeline(
                 QueryPlanOptimizationSettings::fromContext(getContext()),
                 BuildQueryPipelineSettings::fromContext(getContext()));
 
-            if (settings.graph)
-            {
-                /// Pipe holds QueryPlan, should not go out-of-scope
-                auto pipe = QueryPipelineBuilder::getPipe(std::move(*pipeline));
-                const auto & processors = pipe.getProcessors();
-
-                if (settings.compact)
-                    printPipelineCompact(processors, buf, settings.query_pipeline_options.header);
-                else
-                    printPipeline(processors, buf);
-            }
-            else
-            {
-                plan.explainPipeline(buf, settings.query_pipeline_options);
-            }
+            if (settings.optimize)
+                plan.optimize(QueryPlanOptimizationSettings::fromContext(getContext()));
+            plan.explainEstimate(res_columns);
+            insert_buf = false;
+            break;
         }
-        else if (dynamic_cast<const ASTInsertQuery *>(ast.getExplainedQuery().get()))
+        case ASTExplainQuery::TableOverride:
         {
-            InterpreterInsertQuery insert(ast.getExplainedQuery(), getContext());
-            auto io = insert.execute();
-            printPipeline(io.pipeline.getProcessors(), buf);
+            if (auto * table_function = ast.getTableFunction()->as<ASTFunction>(); !table_function || table_function->name != "mysql")
+            {
+                throw Exception(ErrorCodes::INCORRECT_QUERY, "EXPLAIN TABLE OVERRIDE is not supported for the {}() table function", table_function->name);
+            }
+            auto storage = getContext()->getQueryContext()->executeTableFunction(ast.getTableFunction());
+            auto metadata_snapshot = storage->getInMemoryMetadata();
+            TableOverrideAnalyzer::Result override_info;
+            TableOverrideAnalyzer override_analyzer(ast.getTableOverride());
+            override_analyzer.analyze(metadata_snapshot, override_info);
+            override_info.appendTo(buf);
+            break;
         }
-        else
-            throw Exception("Only SELECT and INSERT is supported for EXPLAIN PIPELINE query", ErrorCodes::INCORRECT_QUERY);
     }
-    else if (ast.getKind() == ASTExplainQuery::QueryEstimates)
-    {
-        if (!dynamic_cast<const ASTSelectWithUnionQuery *>(ast.getExplainedQuery().get()))
-            throw Exception("Only SELECT is supported for EXPLAIN ESTIMATE query", ErrorCodes::INCORRECT_QUERY);
-
-        auto settings = checkAndGetSettings<QueryPlanSettings>(ast.getSettings());
-        QueryPlan plan;
-
-        InterpreterSelectWithUnionQuery interpreter(ast.getExplainedQuery(), getContext(), SelectQueryOptions());
-        interpreter.buildQueryPlan(plan);
-        // collect the selected marks, rows, parts during build query pipeline.
-        plan.buildQueryPipeline(
-            QueryPlanOptimizationSettings::fromContext(getContext()),
-            BuildQueryPipelineSettings::fromContext(getContext()));
-
-        if (settings.optimize)
-            plan.optimize(QueryPlanOptimizationSettings::fromContext(getContext()));
-        plan.explainEstimate(res_columns);
-    }
-    if (ast.getKind() != ASTExplainQuery::QueryEstimates)
+    if (insert_buf)
     {
         if (single_line)
             res_columns[0]->insertData(buf.str().data(), buf.str().size());

src/Interpreters/TableOverrideUtils.cpp

@@ -0,0 +1,174 @@
+#include <Interpreters/TableOverrideUtils.h>
+
+#include <Common/quoteString.h>
+#include <DataTypes/IDataType.h>
+#include <DataTypes/DataTypeFactory.h>
+#include <IO/WriteBufferFromString.h>
+#include <Interpreters/InDepthNodeVisitor.h>
+#include <Interpreters/RequiredSourceColumnsVisitor.h>
+#include <Parsers/ASTColumnDeclaration.h>
+#include <Parsers/ASTCreateQuery.h>
+#include <Parsers/ASTFunction.h>
+#include <Parsers/ASTTableOverrides.h>
+#include <Parsers/IAST.h>
+#include <Storages/ColumnDefault.h>
+#include <Storages/StorageInMemoryMetadata.h>
+
+namespace DB
+{
+
+namespace ErrorCodes
+{
+    extern const int INVALID_TABLE_OVERRIDE;
+}
+
+namespace
+{
+
+class MaybeNullableColumnsMatcher
+{
+public:
+    using Visitor = ConstInDepthNodeVisitor<MaybeNullableColumnsMatcher, false>;
+    using Data = RequiredSourceColumnsData;
+
+    static bool needChildVisit(const ASTPtr & node, const ASTPtr & child)
+    {
+        if (const auto * f = node->as<ASTFunction>(); f && f->name == "assumeNotNull")
+            return false;
+        return RequiredSourceColumnsMatcher::needChildVisit(node, child);
+    }
+
+    static void visit(const ASTPtr & ast, Data & data)
+    {
+        RequiredSourceColumnsMatcher::visit(ast, data);
+    }
+};
+
+using MaybeNullableColumnsVisitor = MaybeNullableColumnsMatcher::Visitor;
+
+}
+
+static void checkRequiredColumns(const IAST * ast, const NameToTypeMap & existing_types, NamesAndTypes & used_columns, const String & what, bool allow_nulls = false)
+{
+    if (!ast)
+        return;
+    RequiredSourceColumnsData columns_data;
+    RequiredSourceColumnsVisitor(columns_data).visit(ast->clone());
+    auto required_columns = columns_data.requiredColumns();
+    for (const auto & column : required_columns)
+    {
+        auto type = existing_types.find(column);
+        if (type == existing_types.end())
+            throw Exception(ErrorCodes::INVALID_TABLE_OVERRIDE, "{} override refers to unknown column {}", what, backQuote(column));
+    }
+    if (!allow_nulls)
+    {
+        RequiredSourceColumnsData nullable_data;
+        MaybeNullableColumnsVisitor(nullable_data).visit(ast->clone());
+        for (const auto & column : nullable_data.requiredColumns())
+        {
+            if (existing_types.find(column)->second->isNullable())
+                throw Exception(
+                    ErrorCodes::INVALID_TABLE_OVERRIDE,
+                    "{} override refers to nullable column {} (use assumeNotNull() if the column does not in fact contain NULL values)",
+                    what,
+                    backQuote(column));
+        }
+    }
+    for (const auto & col : required_columns)
+    {
+        used_columns.push_back({col, existing_types.find(col)->second});
+    }
+}
+
+void TableOverrideAnalyzer::analyze(const StorageInMemoryMetadata & metadata, Result & result) const
+{
+    for (const auto & column : metadata.columns)
+        result.existing_types[column.name] = column.type;
+    checkRequiredColumns(override->storage->order_by, result.existing_types, result.order_by_columns, "ORDER BY");
+    checkRequiredColumns(override->storage->primary_key, result.existing_types, result.primary_key_columns, "PRIMARY KEY");
+    checkRequiredColumns(override->storage->partition_by, result.existing_types, result.partition_by_columns, "PARTITION BY");
+    checkRequiredColumns(override->storage->sample_by, result.existing_types, result.sample_by_columns, "SAMPLE BY");
+    checkRequiredColumns(override->storage->ttl_table, result.existing_types, result.ttl_columns, "TTL");
+    if (override->columns && override->columns->columns)
+    {
+        for (const auto & column_ast : override->columns->columns->children)
+        {
+            auto * override_column = column_ast->as<ASTColumnDeclaration>();
+            auto override_type = DataTypeFactory::instance().get(override_column->type);
+            auto found = metadata.columns.tryGetColumnOrSubcolumn(ColumnsDescription::GetFlags::All, override_column->name);
+            std::optional<ColumnDefaultKind> override_default_kind;
+            if (!override_column->default_specifier.empty())
+                override_default_kind = columnDefaultKindFromString(override_column->default_specifier);
+            if (found)
+            {
+                std::optional<ColumnDefaultKind> existing_default_kind;
+                if (auto col_default = metadata.columns.getDefault(found->name))
+                    existing_default_kind = col_default->kind;
+                if (existing_default_kind != override_default_kind)
+                    throw Exception(ErrorCodes::INVALID_TABLE_OVERRIDE, "column {}: modifying default specifier is not allowed", backQuote(override_column->name));
+                result.modified_columns.push_back({found->name, override_type});
+                /// TODO: validate that the original type can be converted to the overridden type
+            }
+            else
+            {
+                if (override_default_kind && *override_default_kind == ColumnDefaultKind::Alias)
+                    result.added_columns.push_back({override_column->name, override_type});
+                else
+                    throw Exception(ErrorCodes::INVALID_TABLE_OVERRIDE, "column {}: can only add ALIAS columns", backQuote(override_column->name));
+            }
+            /// TODO: validate default and materialized expressions (use checkRequiredColumns, allowing nulls)
+        }
+    }
+}
+
+void TableOverrideAnalyzer::Result::appendTo(WriteBuffer & ostr)
+{
+    const auto & format_names = [&](const NamesAndTypes & names) -> String
+    {
+        WriteBufferFromOwnString buf;
+        bool first = true;
+        for (const auto & name : names)
+        {
+            if (!first)
+                buf << ", ";
+            first = false;
+            buf << backQuote(name.name) << " ";
+            auto old_type = existing_types.find(name.name);
+            if (old_type != existing_types.end() && old_type->second != name.type)
+                buf << old_type->second->getName() << " -> ";
+            buf << name.type->getName();
+        }
+        return buf.str();
+    };
+    if (!modified_columns.empty())
+    {
+        ostr << "Modified columns: " << format_names(modified_columns) << "\n";
+    }
+    if (!added_columns.empty())
+    {
+        ostr << "Added columns: " << format_names(added_columns) << "\n";
+    }
+    if (!order_by_columns.empty())
+    {
+        ostr << "ORDER BY uses columns: " << format_names(order_by_columns) << "\n";
+    }
+    if (!primary_key_columns.empty())
+    {
+        ostr << "PRIMARY KEY uses columns: " << format_names(primary_key_columns) << "\n";
+    }
+    if (!partition_by_columns.empty())
+    {
+        ostr << "PARTITION BY uses columns: " << format_names(partition_by_columns) << "\n";
+    }
+    if (!sample_by_columns.empty())
+    {
+        ostr << "SAMPLE BY uses columns: " << format_names(sample_by_columns) << "\n";
+    }
+    if (!ttl_columns.empty())
+    {
+        ostr << "TTL uses columns: " << format_names(ttl_columns) << "\n";
+    }
+}
+
+}

src/Interpreters/TableOverrideUtils.h

@@ -0,0 +1,38 @@
+#pragma once
+
+#include <Common/assert_cast.h>
+#include <Core/Names.h>
+#include <Core/NamesAndTypes.h>
+#include <Parsers/ASTTableOverrides.h>
+
+namespace DB
+{
+
+struct StorageInMemoryMetadata;
+
+using NameToTypeMap = std::map<String, DataTypePtr>;
+
+struct TableOverrideAnalyzer
+{
+    struct Result
+    {
+        NameToTypeMap existing_types;
+        NamesAndTypes order_by_columns;
+        NamesAndTypes primary_key_columns;
+        NamesAndTypes partition_by_columns;
+        NamesAndTypes sample_by_columns;
+        NamesAndTypes ttl_columns;
+        NamesAndTypes added_columns;
+        NamesAndTypes modified_columns;
+
+        void appendTo(WriteBuffer &);
+    };
+
+    ASTTableOverride * override;
+
+    explicit TableOverrideAnalyzer(ASTPtr ast) : override(assert_cast<ASTTableOverride *>(ast.get())) { }
+
+    void analyze(const StorageInMemoryMetadata & metadata, Result & result) const;
+};
+
+}

src/Parsers/ASTExplainQuery.h

@@ -18,6 +18,7 @@ public:
         QueryPlan, /// 'EXPLAIN SELECT ...'
         QueryPipeline, /// 'EXPLAIN PIPELINE ...'
         QueryEstimates, /// 'EXPLAIN ESTIMATE ...'
+        TableOverride, /// 'EXPLAIN TABLE OVERRIDE ...'
     };
 
     explicit ASTExplainQuery(ExplainKind kind_) : kind(kind_) {}
@@ -45,8 +46,22 @@ public:
         ast_settings = std::move(settings_);
     }
 
+    void setTableFunction(ASTPtr table_function_)
+    {
+        children.emplace_back(table_function_);
+        table_function = std::move(table_function_);
+    }
+
+    void setTableOverride(ASTPtr table_override_)
+    {
+        children.emplace_back(table_override_);
+        table_override = std::move(table_override_);
+    }
+
     const ASTPtr & getExplainedQuery() const { return query; }
     const ASTPtr & getSettings() const { return ast_settings; }
+    const ASTPtr & getTableFunction() const { return table_function; }
+    const ASTPtr & getTableOverride() const { return table_override; }
 
 protected:
     void formatQueryImpl(const FormatSettings & settings, FormatState & state, FormatStateStacked frame) const override
@@ -59,8 +74,21 @@ protected:
             ast_settings->formatImpl(settings, state, frame);
         }
 
-        settings.ostr << settings.nl_or_ws;
-        query->formatImpl(settings, state, frame);
+        if (query)
+        {
+            settings.ostr << settings.nl_or_ws;
+            query->formatImpl(settings, state, frame);
+        }
+        if (table_function)
+        {
+            settings.ostr << settings.nl_or_ws;
+            table_function->formatImpl(settings, state, frame);
+        }
+        if (table_override)
+        {
+            settings.ostr << settings.nl_or_ws;
+            table_override->formatImpl(settings, state, frame);
+        }
     }
 
 private:
@@ -69,6 +97,10 @@ private:
     ASTPtr query;
     ASTPtr ast_settings;
 
+    /// Used by EXPLAIN TABLE OVERRIDE
+    ASTPtr table_function;
+    ASTPtr table_override;
+
     static String toString(ExplainKind kind)
     {
         switch (kind)
@@ -78,6 +110,7 @@ private:
             case QueryPlan: return "EXPLAIN";
             case QueryPipeline: return "EXPLAIN PIPELINE";
             case QueryEstimates: return "EXPLAIN ESTIMATE";
+            case TableOverride: return "EXPLAIN TABLE OVERRIDE";
         }
 
         __builtin_unreachable();

src/Parsers/ASTTableOverrides.cpp

@@ -31,13 +31,19 @@ void ASTTableOverride::formatImpl(const FormatSettings & settings_, FormatState
     String hl_keyword = settings.hilite ? hilite_keyword : "";
     String hl_none = settings.hilite ? hilite_none : "";
 
-    settings.ostr << hl_keyword << "TABLE OVERRIDE " << hl_none;
-    ASTIdentifier(table_name).formatImpl(settings, state, frame);
+    if (is_standalone)
+    {
+        settings.ostr << hl_keyword << "TABLE OVERRIDE " << hl_none;
+        ASTIdentifier(table_name).formatImpl(settings, state, frame);
+    }
     if (!columns && (!storage || storage->children.empty()))
         return;
     auto override_frame = frame;
-    ++override_frame.indent;
-    settings.ostr << nl_or_ws << '(' << nl_or_nothing;
+    if (is_standalone)
+    {
+        ++override_frame.indent;
+        settings.ostr << nl_or_ws << '(' << nl_or_nothing;
+    }
     String indent_str = settings.one_line ? "" : String(4 * override_frame.indent, ' ');
     size_t override_elems = 0;
     if (columns)
@@ -68,7 +74,8 @@ void ASTTableOverride::formatImpl(const FormatSettings & settings_, FormatState
         format_storage_elem(storage->ttl_table, "TTL");
     }
 
-    settings.ostr << nl_or_nothing << ')';
+    if (is_standalone)
+        settings.ostr << nl_or_nothing << ')';
 }
 
 ASTPtr ASTTableOverrideList::clone() const

src/Parsers/ASTTableOverrides.h

@@ -15,7 +15,7 @@ class ASTStorage;
 
 /// Storage and column overrides for a single table, for example:
 ///
-///   TABLE OVERRIDE `foo` PARTITION BY toYYYYMM(`createtime`)
+///   TABLE OVERRIDE `foo` (PARTITION BY toYYYYMM(`createtime`))
 ///
 class ASTTableOverride : public IAST
 {
@@ -23,6 +23,7 @@ public:
     String table_name;
     ASTColumns * columns = nullptr;
     ASTStorage * storage = nullptr;
+    bool is_standalone = true;
     String getID(char) const override { return "TableOverride " + table_name; }
     ASTPtr clone() const override;
     void formatImpl(const FormatSettings & settings, FormatState & state, FormatStateStacked frame) const override;

src/Parsers/ParserCreateQuery.cpp

@@ -969,14 +969,15 @@ bool ParserTableOverrideDeclaration::parseImpl(Pos & pos, ASTPtr & node, Expecte
     ASTPtr sample_by;
     ASTPtr ttl_table;
 
-    if (!s_table_override.ignore(pos, expected))
-        return false;
-
-    if (!table_name_p.parse(pos, table_name, expected))
-        return false;
-
-    if (!lparen_p.ignore(pos, expected))
-        return false;
+    if (is_standalone)
+    {
+        if (!s_table_override.ignore(pos, expected))
+            return false;
+        if (!table_name_p.parse(pos, table_name, expected))
+            return false;
+        if (!lparen_p.ignore(pos, expected))
+            return false;
+    }
 
     while (true)
     {
@@ -1034,7 +1035,7 @@ bool ParserTableOverrideDeclaration::parseImpl(Pos & pos, ASTPtr & node, Expecte
         break;
     }
 
-    if (!rparen_p.ignore(pos, expected))
+    if (is_standalone && !rparen_p.ignore(pos, expected))
         return false;
 
     auto storage = std::make_shared<ASTStorage>();
@@ -1045,7 +1046,9 @@ bool ParserTableOverrideDeclaration::parseImpl(Pos & pos, ASTPtr & node, Expecte
     storage->set(storage->ttl_table, ttl_table);
 
     auto res = std::make_shared<ASTTableOverride>();
-    res->table_name = table_name->as<ASTIdentifier>()->name();
+    if (table_name)
+        res->table_name = table_name->as<ASTIdentifier>()->name();
+    res->is_standalone = is_standalone;
     res->set(res->storage, storage);
     if (columns)
         res->set(res->columns, columns);

src/Parsers/ParserCreateQuery.h

@@ -389,6 +389,10 @@ protected:
 
 class ParserTableOverrideDeclaration : public IParserBase
 {
+public:
+    const bool is_standalone;
+    ParserTableOverrideDeclaration(bool is_standalone_ = true) : is_standalone(is_standalone_) { }
+
 protected:
     const char * getName() const override { return "table override declaration"; }
     bool parseImpl(Pos & pos, ASTPtr & node, Expected & expected) override;

src/Parsers/ParserExplainQuery.cpp

@@ -21,6 +21,7 @@ bool ParserExplainQuery::parseImpl(Pos & pos, ASTPtr & node, Expected & expected
     ParserKeyword s_pipeline("PIPELINE");
     ParserKeyword s_plan("PLAN");
     ParserKeyword s_estimates("ESTIMATE");
+    ParserKeyword s_table_override("TABLE OVERRIDE");
 
     if (s_explain.ignore(pos, expected))
     {
@@ -36,6 +37,8 @@ bool ParserExplainQuery::parseImpl(Pos & pos, ASTPtr & node, Expected & expected
             kind = ASTExplainQuery::ExplainKind::QueryPlan; //-V1048
         else if (s_estimates.ignore(pos, expected))
             kind = ASTExplainQuery::ExplainKind::QueryEstimates; //-V1048
+        else if (s_table_override.ignore(pos, expected))
+            kind = ASTExplainQuery::ExplainKind::TableOverride;
     }
     else
         return false;
@@ -65,6 +68,17 @@ bool ParserExplainQuery::parseImpl(Pos & pos, ASTPtr & node, Expected & expected
         else
             return false;
     }
+    else if (kind == ASTExplainQuery::ExplainKind::TableOverride)
+    {
+        ASTPtr table_function;
+        if (!ParserFunction(true, true).parse(pos, table_function, expected))
+            return false;
+        ASTPtr table_override;
+        if (!ParserTableOverrideDeclaration(false).parse(pos, table_override, expected))
+            return false;
+        explain_query->setTableFunction(table_function);
+        explain_query->setTableOverride(table_override);
+    }
     else if (select_p.parse(pos, query, expected) ||
         create_p.parse(pos, query, expected) ||
         insert_p.parse(pos, query, expected))

tests/integration/test_materialized_mysql_database/materialize_with_ddl.py

@@ -1079,9 +1079,41 @@ def table_overrides(clickhouse_node, mysql_node, service_name):
     check_query(clickhouse_node, "SELECT count() FROM table_overrides.t1", "1000\n")
     mysql_node.query("INSERT INTO table_overrides.t1 VALUES(1001, '2021-10-01 00:00:00', 42.0)")
     check_query(clickhouse_node, "SELECT count() FROM table_overrides.t1", "1001\n")
+
+    explain_with_table_func = f"EXPLAIN TABLE OVERRIDE mysql('{service_name}:3306', 'table_overrides', 't1', 'root', 'clickhouse')"
+
+    for what in ['ORDER BY', 'PRIMARY KEY', 'SAMPLE BY', 'PARTITION BY', 'TTL']:
+        with pytest.raises(QueryRuntimeException) as exc:
+            clickhouse_node.query(f"{explain_with_table_func} {what} temperature")
+        assert f'{what} override refers to nullable column `temperature`' in \
+            str(exc.value)
+        assert f"{what} uses columns: `temperature` Nullable(Float32)" in \
+            clickhouse_node.query(f"{explain_with_table_func} {what} assumeNotNull(temperature)")
+
+    for testcase in [
+        ('COLUMNS (temperature Nullable(Float32) MATERIALIZED 1.0)',
+         'column `temperature`: modifying default specifier is not allowed'),
+        ('COLUMNS (sensor_id UInt64 ALIAS 42)',
+         'column `sensor_id`: modifying default specifier is not allowed')
+    ]:
+        with pytest.raises(QueryRuntimeException) as exc:
+            clickhouse_node.query(f"{explain_with_table_func} {testcase[0]}")
+        assert testcase[1] in str(exc.value)
+
+    for testcase in [
+        ('COLUMNS (temperature Nullable(Float64))',
+         'Modified columns: `temperature` Nullable(Float32) -> Nullable(Float64)'),
+        ('COLUMNS (temp_f Nullable(Float32) ALIAS if(temperature IS NULL, NULL, (temperature * 9.0 / 5.0) + 32),\
+                   temp_k Nullable(Float32) ALIAS if(temperature IS NULL, NULL, temperature + 273.15))',
+         'Added columns: `temp_f` Nullable(Float32), `temp_k` Nullable(Float32)')
+    ]:
+        assert testcase[1] in clickhouse_node.query(
+            f"{explain_with_table_func} {testcase[0]}")
+
     clickhouse_node.query("DROP DATABASE IF EXISTS table_overrides")
     mysql_node.query("DROP DATABASE IF EXISTS table_overrides")
 
+
 def materialized_database_support_all_kinds_of_mysql_datatype(clickhouse_node, mysql_node, service_name):
     mysql_node.query("DROP DATABASE IF EXISTS test_database_datatype")
     clickhouse_node.query("DROP DATABASE IF EXISTS test_database_datatype")