Improve decompression in readbig to avoid unnecessary memcpy (#36095)

src/Compression/CompressedReadBufferFromFile.cpp

@@ -117,6 +117,25 @@ size_t CompressedReadBufferFromFile::readBig(char * to, size_t n)
             bytes_read += size_decompressed;
             bytes += size_decompressed;
         }
+        else if (nextimpl_working_buffer_offset > 0)
+        {
+            /// Need to skip some bytes in decompressed data (seek happened before readBig call).
+            size_compressed = new_size_compressed;
+            bytes += offset();
+
+            /// This is for clang static analyzer.
+            assert(size_decompressed + additional_size_at_the_end_of_buffer > 0);
+            memory.resize(size_decompressed + additional_size_at_the_end_of_buffer);
+            working_buffer = Buffer(memory.data(), &memory[size_decompressed]);
+            decompress(working_buffer, size_decompressed, size_compressed_without_checksum);
+
+            /// Read partial data from first block. Won't run here at second block.
+            /// Avoid to call nextImpl and unnecessary memcpy in read when the second block fits entirely to output buffer.
+            size_t size_partial = std::min((size_decompressed - nextimpl_working_buffer_offset),(n - bytes_read));
+            pos = working_buffer.begin() + nextimpl_working_buffer_offset;
+            nextimpl_working_buffer_offset = 0;
+            bytes_read += read(to + bytes_read, size_partial);
+        }
         else
         {
             size_compressed = new_size_compressed;
@@ -124,17 +143,12 @@ size_t CompressedReadBufferFromFile::readBig(char * to, size_t n)
 
             /// This is for clang static analyzer.
             assert(size_decompressed + additional_size_at_the_end_of_buffer > 0);
-
             memory.resize(size_decompressed + additional_size_at_the_end_of_buffer);
             working_buffer = Buffer(memory.data(), &memory[size_decompressed]);
-
             decompress(working_buffer, size_decompressed, size_compressed_without_checksum);
 
-            /// Manually take nextimpl_working_buffer_offset into account, because we don't use
+            ///Read partial data from last block.
-            /// nextImpl in this method.
+            pos = working_buffer.begin();
-            pos = working_buffer.begin() + nextimpl_working_buffer_offset;
-            nextimpl_working_buffer_offset = 0;
-
             bytes_read += read(to + bytes_read, n - bytes_read);
             break;
         }