Fix query result decoding - decode binary packets into structured data - #2

Problem

Query results were returned as raw binary packets, making the library difficult to use:

% Before - raw binary that users had to decode manually
{ok, <<1,0,1,0,2,255,255,255,255,0,2,1,5>>}

Users had to understand ClickHouse protocol internals to decode these packets, which created a poor developer experience.

Solution

Implemented automatic query result decoding that transforms binary packets into structured, user-friendly data:

% After - decoded structured data ready for use
{ok, [{data,#{block_info => #{num_columns => 2,num_rows => 1}}}, end_of_stream]}

Changes Made

Core Implementation

• Added decode_query_result/1 in clicker_protocol.erl - Main decoder that handles multiple packet types
• Added packet-specific decoders:
  • decode_data_packet/1 - Extracts column/row information from data blocks
  • decode_exception_packet/1 - Parses error details (code, message, stack trace)
  • decode_progress_packet/1 - Extracts query progress information
• Updated query handling in clicker_conn.erl to automatically decode responses

Supported Packet Types

• Data packets: Include block metadata (number of columns and rows)
• Exception packets: Provide detailed error information
• Progress packets: Show query execution progress
• End of stream packets: Signal query completion
• Unknown packets: Handled gracefully without breaking

Examples

% Successful query with data
{ok, [{data, #{block_info => #{num_columns => 2, num_rows => 10}}}, end_of_stream]}

% Query with error
{ok, [{exception, #{code => 404, name => "NotFound", message => "Table not found", stack_trace => ""}}]}

% Query with progress updates
{ok, [{progress, #{read_rows => 1000, read_bytes => 8000, total_rows => 5000}}, 
      {data, #{block_info => #{num_columns => 3, num_rows => 1000}}}, 
      end_of_stream]}

Testing

• ✅ Comprehensive unit tests for all packet types
• ✅ Error handling validation
• ✅ Multi-packet sequence decoding
• ✅ Updated integration tests

Benefits

• User-friendly: No manual binary decoding required
• Robust: Built-in error handling and validation
• Informative: Clear indication of data, errors, and progress
• Backward compatible: Same API, better results

Fixes #1.

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