diff --git a/examples/memtest.py b/examples/memtest.py
new file mode 100644
index 000000000..8a63090e8
--- /dev/null
+++ b/examples/memtest.py
@@ -0,0 +1,32 @@
+# %%
+import psutil, os, time, threading
+PEAK_MEM_USAGE = 0
+SELF_PROC = psutil.Process(os.getpid())
+
+def track_mem():
+    global PEAK_MEM_USAGE
+    while True:
+        m = SELF_PROC.memory_info().rss
+        if m > PEAK_MEM_USAGE:
+            PEAK_MEM_USAGE = m
+        time.sleep(0.1)
+
+threading.Thread(target=track_mem, daemon=True).start()
+print(f"[MEM] start={PEAK_MEM_USAGE / (1024**3)} GB")
+
+# %%
+from pyhealth.datasets import MIMIC4Dataset
+DATASET_DIR = "/home/logic/physionet.org/files/mimiciv/3.1"
+dataset = MIMIC4Dataset(
+    ehr_root=DATASET_DIR,
+    ehr_tables=[
+        "patients",
+        "admissions",
+        "diagnoses_icd",
+        "procedures_icd",
+        "prescriptions",
+        "labevents",
+    ],
+)
+print(f"[MEM] __init__={PEAK_MEM_USAGE / (1024**3):.3f} GB")
+# %%
diff --git a/pyhealth/datasets/base_dataset.py b/pyhealth/datasets/base_dataset.py
index 43948c828..3390453ff 100644
--- a/pyhealth/datasets/base_dataset.py
+++ b/pyhealth/datasets/base_dataset.py
@@ -208,6 +208,12 @@ def load_table(self, table_name: str) -> pl.LazyFrame:
         if table_name not in self.config.tables:
             raise ValueError(f"Table {table_name} not found in config")
 
+        def _to_lower(col_name: str) -> str:
+            lower_name = col_name.lower()
+            if lower_name != col_name:
+                logger.warning("Renaming column %s to lowercase %s", col_name, lower_name)
+            return lower_name
+
         table_cfg = self.config.tables[table_name]
         csv_path = f"{self.root}/{table_cfg.file_path}"
         csv_path = clean_path(csv_path)
@@ -216,10 +222,7 @@ def load_table(self, table_name: str) -> pl.LazyFrame:
         df = scan_csv_gz_or_csv_tsv(csv_path)
 
         # Convert column names to lowercase before calling preprocess_func
-        col_names = df.collect_schema().names()
-        if any(col != col.lower() for col in col_names):
-            logger.warning("Some column names were converted to lowercase")
-        df = df.with_columns([pl.col(col).alias(col.lower()) for col in col_names])
+        df = df.rename(_to_lower)
 
         # Check if there is a preprocessing function for this table
         preprocess_func = getattr(self, f"preprocess_{table_name}", None)
@@ -235,12 +238,7 @@ def load_table(self, table_name: str) -> pl.LazyFrame:
             other_csv_path = clean_path(other_csv_path)
             logger.info(f"Joining with table: {other_csv_path}")
             join_df = scan_csv_gz_or_csv_tsv(other_csv_path)
-            join_df = join_df.with_columns(
-                [
-                    pl.col(col).alias(col.lower())
-                    for col in join_df.collect_schema().names()
-                ]
-            )
+            join_df = join_df.rename(_to_lower)
             join_key = join_cfg.on
             columns = join_cfg.columns
             how = join_cfg.how
@@ -283,7 +281,7 @@ def load_table(self, table_name: str) -> pl.LazyFrame:
 
         # Flatten attribute columns with event_type prefix
         attribute_columns = [
-            pl.col(attr).alias(f"{table_name}/{attr}") for attr in attribute_cols
+            pl.col(attr.lower()).alias(f"{table_name}/{attr}") for attr in attribute_cols
         ]
 
         event_frame = df.select(base_columns + attribute_columns)