5. Data Flow

A SQL query is essentially a series of transformations on some input data. While the input to the query engine is a SQL query, the query engine (and more so, the Executor) can be thought of a black box which builds a transformation, loads data, applies the transformation, and returns the result. In the following section, we provide a summary of how data flows through the system, from physical storage on disk through the memory hierarchy to the output.

5.1. Input Data

All requests for input data start from the Executor. The executor loads chunks required for a query to device memory by making a call into the standalone ColumnFetcher class. The ColumnFetcher makes calls directly into the buffer manager hierarchy and storage layer to ensure requested chunks are available in the appropriate memory level for the device.

The following schematic illustrates the process for requesting inputs for a query step, from storage through to the execution device (in this example, a GPU).

@startuml "Executor::fetchChunks" -> "Column Fetcher": Request Chunks for Query alt input is physical table "Column Fetcher" -> Chunk: Build Input Chunk Chunk -> "Data Mgr": Request Chunk Buffer "Data Mgr" -> "GPU Buffer Mgr": Request Chunk Buffer "GPU Buffer Mgr" -> "CPU Buffer Mgr": Request Chunk Buffer "CPU Buffer Mgr" -> "File Mgr": Request Chunk Buffer else input is intermediate result "Column Fetcher" -> "Data Mgr": Convert Temporary Buffer to Input and Load to GPU end @enduml

For a physical table, input data is loaded from the storage layer (see Physical Data Layout and Columnar Data Organization) via the buffer manager hierarchy (see Memory Layout). If the data is already present on GPU, the request terminates with the GPU Buffer Mgr. If not, the request passes on to the parent manager until it reaches storage.

For intermediate results, input data is loaded directly from a per-query temporary tables map and transferred to the GPU directly via the Data Mgr.

5.2. Output Data

Query step outputs are managed by the ResultSet (see Query Results). However, some intermediate buffers may still be required after a query completes. The RowSetMemoryOwner is a helper structure for managing the lifetime of all outputs related to a given query. The ResultSet holds a shared pointer to the RowSetMemoryOwner, ensuring that the data held by a ResultSet is always valid until the ResultSet class is destroyed.