community[minor]: Add tidb loader support (#17788)

This pull request support loading data from TiDB database with
Langchain.

A simple usage:
```
from  langchain_community.document_loaders import TiDBLoader

CONNECTION_STRING = "mysql+pymysql://root@127.0.0.1:4000/test"

QUERY = "select id, name, description from items;"
loader = TiDBLoader(
    connection_string=CONNECTION_STRING,
    query=QUERY,
    page_content_columns=["name", "description"],
    metadata_columns=["id"],
)
documents = loader.load()
print(documents)
```

libs/community/langchain_community/document_loaders/__init__.py

@@ -199,6 +199,7 @@ from langchain_community.document_loaders.tensorflow_datasets import (
     TensorflowDatasetLoader,
 )
 from langchain_community.document_loaders.text import TextLoader
+from langchain_community.document_loaders.tidb import TiDBLoader
 from langchain_community.document_loaders.tomarkdown import ToMarkdownLoader
 from langchain_community.document_loaders.toml import TomlLoader
 from langchain_community.document_loaders.trello import TrelloLoader
@@ -380,6 +381,7 @@ __all__ = [
     "TencentCOSDirectoryLoader",
     "TencentCOSFileLoader",
     "TextLoader",
+    "TiDBLoader",
     "ToMarkdownLoader",
     "TomlLoader",
     "TrelloLoader",

libs/community/langchain_community/document_loaders/tidb.py

@@ -0,0 +1,71 @@
+from typing import Any, Dict, Iterator, List, Optional
+
+from langchain_core.documents import Document
+
+from langchain_community.document_loaders.base import BaseLoader
+
+
+class TiDBLoader(BaseLoader):
+    """Load documents from TiDB."""
+
+    def __init__(
+        self,
+        connection_string: str,
+        query: str,
+        page_content_columns: Optional[List[str]] = None,
+        metadata_columns: Optional[List[str]] = None,
+        engine_args: Optional[Dict[str, Any]] = None,
+    ) -> None:
+        """Initialize TiDB document loader.
+
+        Args:
+            connection_string (str): The connection string for the TiDB database,
+                format: "mysql+pymysql://root@127.0.0.1:4000/test".
+            query: The query to run in TiDB.
+            page_content_columns: Optional. Columns written to Document `page_content`,
+                default(None) to all columns.
+            metadata_columns: Optional. Columns written to Document `metadata`,
+                default(None) to no columns.
+            engine_args: Optional. Additional arguments to pass to sqlalchemy engine.
+        """
+        self.connection_string = connection_string
+        self.query = query
+        self.page_content_columns = page_content_columns
+        self.metadata_columns = metadata_columns if metadata_columns is not None else []
+        self.engine_args = engine_args
+
+    def lazy_load(self) -> Iterator[Document]:
+        """Lazy load TiDB data into document objects."""
+
+        from sqlalchemy import create_engine
+        from sqlalchemy.engine import Engine
+        from sqlalchemy.sql import text
+
+        # use sqlalchemy to create db connection
+        engine: Engine = create_engine(
+            self.connection_string, **(self.engine_args or {})
+        )
+
+        # execute query
+        with engine.connect() as conn:
+            result = conn.execute(text(self.query))
+
+            # convert result to Document objects
+            column_names = list(result.keys())
+            for row in result:
+                # convert row to dict{column:value}
+                row_data = {
+                    column_names[index]: value for index, value in enumerate(row)
+                }
+                page_content = "\n".join(
+                    f"{k}: {v}"
+                    for k, v in row_data.items()
+                    if self.page_content_columns is None
+                    or k in self.page_content_columns
+                )
+                metadata = {col: row_data[col] for col in self.metadata_columns}
+                yield Document(page_content=page_content, metadata=metadata)
+
+    def load(self) -> List[Document]:
+        """Load TiDB data into document objects."""
+        return list(self.lazy_load())