feat: Cache SQL columns and schemas (#1779)

* cache sql columns and schemas

* lint fix

* fix mypy typing

* use lru_cache instead of custom cache

* Revert "use lru_cache instead of custom cache"

This reverts commit 668832b.

* use set for cache instead of list

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* fix set typing

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: Ken Payne <ken@meltano.com>
Co-authored-by: Edgar R. M <edgar@meltano.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

singer_sdk/connectors/sql.py

@@ -58,6 +58,8 @@ def __init__(
         """
         self._config: dict[str, t.Any] = config or {}
         self._sqlalchemy_url: str | None = sqlalchemy_url or None
+        self._table_cols_cache: dict[str, dict[str, sqlalchemy.Column]] = {}
+        self._schema_cache: set[str] = set()
 
     @property
     def config(self) -> dict:
@@ -584,8 +586,12 @@ def schema_exists(self, schema_name: str) -> bool:
         Returns:
             True if the database schema exists, False if not.
         """
-        schema_names = sqlalchemy.inspect(self._engine).get_schema_names()
-        return schema_name in schema_names
+        if schema_name not in self._schema_cache:
+            self._schema_cache = set(
+                sqlalchemy.inspect(self._engine).get_schema_names(),
+            )
+
+        return schema_name in self._schema_cache
 
     def get_table_columns(
         self,
@@ -601,20 +607,24 @@ def get_table_columns(
         Returns:
             An ordered list of column objects.
         """
-        _, schema_name, table_name = self.parse_full_table_name(full_table_name)
-        inspector = sqlalchemy.inspect(self._engine)
-        columns = inspector.get_columns(table_name, schema_name)
-
-        return {
-            col_meta["name"]: sqlalchemy.Column(
-                col_meta["name"],
-                col_meta["type"],
-                nullable=col_meta.get("nullable", False),
-            )
-            for col_meta in columns
-            if not column_names
-            or col_meta["name"].casefold() in {col.casefold() for col in column_names}
-        }
+        if full_table_name not in self._table_cols_cache:
+            _, schema_name, table_name = self.parse_full_table_name(full_table_name)
+            inspector = sqlalchemy.inspect(self._engine)
+            columns = inspector.get_columns(table_name, schema_name)
+
+            self._table_cols_cache[full_table_name] = {
+                col_meta["name"]: sqlalchemy.Column(
+                    col_meta["name"],
+                    col_meta["type"],
+                    nullable=col_meta.get("nullable", False),
+                )
+                for col_meta in columns
+                if not column_names
+                or col_meta["name"].casefold()
+                in {col.casefold() for col in column_names}
+            }
+
+        return self._table_cols_cache[full_table_name]
 
     def get_table(
         self,