From 1b03a782b91f97abd2385985e0f89f78548eed16 Mon Sep 17 00:00:00 2001
From: Dmitry Stasenko <dmitry.stasenko@pinely.com>
Date: Thu, 7 Dec 2023 02:39:35 +0300
Subject: [PATCH] Reserve memory in builder before loop. Do not read validity
 bits when we don't have nulls

---
 cpp/src/parquet/column_reader.cc | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/cpp/src/parquet/column_reader.cc b/cpp/src/parquet/column_reader.cc
index ecc48811e46..051deb0259f 100644
--- a/cpp/src/parquet/column_reader.cc
+++ b/cpp/src/parquet/column_reader.cc
@@ -2065,8 +2065,9 @@ class FLBARecordReader : public TypedRecordReader<FLBAType>,
         this->current_decoder_->Decode(values, static_cast<int>(values_to_read));
     CheckNumberDecoded(num_decoded, values_to_read);
 
+    PARQUET_THROW_NOT_OK(builder_->Reserve(num_decoded));
     for (int64_t i = 0; i < num_decoded; i++) {
-      PARQUET_THROW_NOT_OK(builder_->Append(values[i].ptr));
+      builder_->UnsafeAppend(values[i].ptr);
     }
     ResetValues();
   }
@@ -2081,11 +2082,13 @@ class FLBARecordReader : public TypedRecordReader<FLBAType>,
         valid_bits, valid_bits_offset);
     ARROW_DCHECK_EQ(num_decoded, values_to_read);
 
+    PARQUET_THROW_NOT_OK(builder_->Reserve(num_decoded));
     for (int64_t i = 0; i < num_decoded; i++) {
-      if (::arrow::bit_util::GetBit(valid_bits, valid_bits_offset + i)) {
-        PARQUET_THROW_NOT_OK(builder_->Append(values[i].ptr));
+      if (null_count == 0 ||
+          ::arrow::bit_util::GetBit(valid_bits, valid_bits_offset + i)) {
+        builder_->UnsafeAppend(values[i].ptr);
       } else {
-        PARQUET_THROW_NOT_OK(builder_->AppendNull());
+        builder_->UnsafeAppendNull();
       }
     }
     ResetValues();