From 2636affa976f4fa38be1507b1739c5aa8c91eb4d Mon Sep 17 00:00:00 2001
From: Nic Crane <thisisnic@gmail.com>
Date: Thu, 4 Jun 2026 09:54:30 +0100
Subject: [PATCH] Add note about dictionary not helping

---
 python/pyarrow/parquet/core.py | 2 ++
 r/R/parquet.R                  | 4 +++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/python/pyarrow/parquet/core.py b/python/pyarrow/parquet/core.py
index ff880fdcf52c..7fed7d09fe21 100644
--- a/python/pyarrow/parquet/core.py
+++ b/python/pyarrow/parquet/core.py
@@ -769,6 +769,8 @@ def _sanitize_table(table, new_schema, flavor):
     When encoding the column, if the dictionary size is too large, the
     column will fallback to ``PLAIN`` encoding. Specially, ``BOOLEAN`` type
     doesn't support dictionary encoding.
+    On columns with many unique values, dictionary encoding can increase
+    file size and reduce compression effectiveness.
 compression : str or dict, default 'snappy'
     Specify the compression codec, either on a general basis or per-column.
     Valid values: {'NONE', 'SNAPPY', 'GZIP', 'BROTLI', 'LZ4', 'LZ4_RAW', 'ZSTD'}.
diff --git a/r/R/parquet.R b/r/R/parquet.R
index 6415e36b03ce..2fe0fef771f5 100644
--- a/r/R/parquet.R
+++ b/r/R/parquet.R
@@ -106,7 +106,9 @@ read_parquet <- function(
 #' @param compression compression algorithm. Default "snappy". See details.
 #' @param compression_level compression level. Meaning depends on compression
 #'    algorithm
-#' @param use_dictionary logical: use dictionary encoding? Default `TRUE`
+#' @param use_dictionary logical: use dictionary encoding? Default `TRUE`.
+#'    On columns with many unique values, dictionary encoding can increase
+#'    file size and reduce compression effectiveness.
 #' @param write_statistics logical: include statistics? Default `TRUE`
 #' @param data_page_size Set a target threshold for the approximate encoded
 #'    size of data pages within a column chunk (in bytes). Default 1 MiB.