Fix and update docs

docs/en/engines/table-engines/special/url.md

@@ -109,6 +109,7 @@ For partitioning by month, use the `toYYYYMM(date_column)` expression, where `da
 - `_file` — Resource name of the `URL`. Type: `LowCardinalty(String)`.
 - `_size` — Size of the resource in bytes. Type: `Nullable(UInt64)`. If the size is unknown, the value is `NULL`.
 - `_time` — Last modified time of the file. Type: `Nullable(DateTime)`. If the time is unknown, the value is `NULL`.
+- `_headers` - HTTP response headers. Type: `Map(LowCardinality(String), LowCardinality(String))`.
 
 ## Storage Settings {#storage-settings}
 

docs/en/sql-reference/table-functions/url.md

@@ -54,6 +54,7 @@ Character `|` inside patterns is used to specify failover addresses. They are it
 - `_file` — Resource name of the `URL`. Type: `LowCardinalty(String)`.
 - `_size` — Size of the resource in bytes. Type: `Nullable(UInt64)`. If the size is unknown, the value is `NULL`.
 - `_time` — Last modified time of the file. Type: `Nullable(DateTime)`. If the time is unknown, the value is `NULL`.
+- `_headers` - HTTP response headers. Type: `Map(LowCardinality(String), LowCardinality(String))`.
 
 ## Hive-style partitioning {#hive-style-partitioning}
 

src/Storages/StorageURL.cpp

@@ -465,7 +465,7 @@ Chunk StorageURLSource::generate()
                     std::make_shared<DataTypeLowCardinality>(std::make_shared<DataTypeString>()),
                     std::make_shared<DataTypeLowCardinality>(std::make_shared<DataTypeString>()));
 
-                chunk.addColumn(type->createColumnConst(chunk.getNumRows(), http_response_headers));
+                chunk.addColumn(type->createColumnConst(chunk.getNumRows(), http_response_headers)->convertToFullColumnIfConst());
             }
             return chunk;
         }