Support customizable SYMBOL column

QuestDB allows SYMBOL columns to specify:
1. Capacity
2. Cache: ON/OFF

This changeset is adding this to SQLAlchemy. It allows to define columns as this:
`instrument = Column(Symbol(capacity=10000, cache=False))`

src/examples/hello_world.py

@@ -15,7 +15,7 @@ class Signal(Base):
     __table_args__ = (
         qdbc.QDBTableEngine("signal", "ts", qdbc.PartitionBy.HOUR, is_wal=True),
     )
-    source = Column(qdbc.Symbol)
+    source = Column(qdbc.Symbol(capacity=1024, cache=False))
     value = Column(qdbc.Double)
     ts = Column(qdbc.Timestamp, primary_key=True)
 

src/questdb_connect/types.py

@@ -1,3 +1,5 @@
+from typing import Optional
+
 import sqlalchemy
 
 from .common import quote_identifier
@@ -121,9 +123,40 @@ class String(QDBTypeMixin):
 
 
 class Symbol(QDBTypeMixin):
+    """
+    QuestDB SYMBOL type implementation with support for capacity and cache parameters.
+
+    Example usage:
+        source = Column(Symbol(capacity=128, cache=True))
+    """
     __visit_name__ = "SYMBOL"
     type_code = 12
 
+    def __init__(
+            self,
+            capacity: Optional[int] = None,
+            cache: Optional[bool] = None,
+            *args, **kwargs
+    ):
+        super().__init__(*args, **kwargs)
+        self.capacity = capacity
+        self.cache = cache
+
+    def compile(self, dialect=None):
+        params = []
+
+        if self.capacity is not None:
+            params.append(f"CAPACITY {self.capacity}")
+        if self.cache is not None:
+            params.append("CACHE" if self.cache else "NOCACHE")
+
+        if params:
+            return f"{self.__visit_name__} {' '.join(params)}"
+        return self.__visit_name__
+
+    def column_spec(self, column_name):
+        return f"{quote_identifier(column_name)} {self.compile()}"
+
 
 class Long256(QDBTypeMixin):
     __visit_name__ = "LONG256"

tests/test_types.py

@@ -1,6 +1,7 @@
 import re
 
 import questdb_connect as qdbc
+from questdb_connect.common import quote_identifier
 
 
 def test_resolve_type_from_name():
@@ -16,6 +17,110 @@ def test_resolve_type_from_name():
         assert isinstance(g_class(), qdbc.geohash_class(n))
 
 
+def test_symbol_type():
+    # Test basic Symbol without parameters
+    symbol = qdbc.Symbol()
+    assert symbol.__visit_name__ == "SYMBOL"
+    assert symbol.compile() == "SYMBOL"
+    assert symbol.column_spec("test_col") == "\"test_col\" SYMBOL"
+
+    # Test Symbol with capacity
+    symbol_cap = qdbc.Symbol(capacity=128)
+    assert symbol_cap.compile() == "SYMBOL CAPACITY 128"
+    assert symbol_cap.column_spec("test_col") == "\"test_col\" SYMBOL CAPACITY 128"
+
+    # Test Symbol with cache true
+    symbol_cache = qdbc.Symbol(cache=True)
+    assert symbol_cache.compile() == "SYMBOL CACHE"
+    assert symbol_cache.column_spec("test_col") == "\"test_col\" SYMBOL CACHE"
+
+    # Test Symbol with cache false
+    symbol_nocache = qdbc.Symbol(cache=False)
+    assert symbol_nocache.compile() == "SYMBOL NOCACHE"
+    assert symbol_nocache.column_spec("test_col") == "\"test_col\" SYMBOL NOCACHE"
+
+    # Test Symbol with both parameters
+    symbol_full = qdbc.Symbol(capacity=256, cache=True)
+    assert symbol_full.compile() == "SYMBOL CAPACITY 256 CACHE"
+    assert symbol_full.column_spec("test_col") == "\"test_col\" SYMBOL CAPACITY 256 CACHE"
+
+    # Test inheritance and type resolution
+    assert isinstance(symbol, qdbc.QDBTypeMixin)
+    resolved_class = qdbc.resolve_type_from_name("SYMBOL")
+    assert resolved_class.__visit_name__ == "SYMBOL"
+    assert isinstance(symbol, resolved_class)
+
+    # Test that parameters don't affect type resolution
+    symbol_with_params = qdbc.Symbol(capacity=128, cache=True)
+    assert isinstance(symbol_with_params, resolved_class)
+    assert isinstance(resolved_class(), type(symbol_with_params))
+
+
+def test_symbol_backward_compatibility():
+    """Verify that the parametrized Symbol type maintains backward compatibility with older code."""
+    # Test all the ways Symbol type could be previously instantiated
+    symbol1 = qdbc.Symbol
+    symbol2 = qdbc.Symbol()
+
+    # Check that both work in column definitions
+    from sqlalchemy import Column, MetaData, Table
+
+    metadata = MetaData()
+    test_table = Table(
+        'test_table',
+        metadata,
+        Column('old_style1', symbol1),  # Old style: direct class reference
+        Column('old_style2', symbol2),  # Old style: basic instantiation
+    )
+
+    # Verify type resolution still works
+    for column in test_table.columns:
+        # Check inheritance
+        assert isinstance(column.type, qdbc.QDBTypeMixin)
+
+        # Check type resolution
+        resolved_class = qdbc.resolve_type_from_name("SYMBOL")
+        assert isinstance(column.type, resolved_class)
+
+        # Check SQL generation matches old behavior
+        assert column.type.compile() == "SYMBOL"
+        assert column.type.column_spec(column.name) == f"{quote_identifier(column.name)} SYMBOL"
+
+def test_symbol_type_in_column():
+    # Test Symbol type in Column definition
+    from sqlalchemy import Column, MetaData, Table
+
+    metadata = MetaData()
+
+    # Create a test table with different Symbol column variations
+    test_table = Table(
+        'test_table',
+        metadata,
+        Column('basic_symbol', qdbc.Symbol()),
+        Column('symbol_with_capacity', qdbc.Symbol(capacity=128)),
+        Column('symbol_with_cache', qdbc.Symbol(cache=True)),
+        Column('symbol_with_nocache', qdbc.Symbol(cache=False)),
+        Column('symbol_full', qdbc.Symbol(capacity=256, cache=True))
+    )
+
+    # Get the create table SQL (implementation-dependent)
+    # This part might need adjustment based on your actual SQL compilation logic
+    for column in test_table.columns:
+        assert isinstance(column.type, qdbc.Symbol)
+        assert isinstance(column.type, qdbc.QDBTypeMixin)
+
+        if column.name == 'basic_symbol':
+            assert column.type.compile() == "SYMBOL"
+        elif column.name == 'symbol_with_capacity':
+            assert column.type.compile() == "SYMBOL CAPACITY 128"
+        elif column.name == 'symbol_with_cache':
+            assert column.type.compile() == "SYMBOL CACHE"
+        elif column.name == 'symbol_with_nocache':
+            assert column.type.compile() == "SYMBOL NOCACHE"
+        elif column.name == 'symbol_full':
+            assert column.type.compile() == "SYMBOL CAPACITY 256 CACHE"
+
+
 def test_superset_default_mappings():
     default_column_type_mappings = (
         (re.compile("^BOOLEAN$", re.IGNORECASE), qdbc.Boolean),