From a4acac04d125cae2eb8f2358dd196dbe029a813e Mon Sep 17 00:00:00 2001
From: Julien Rouhaud <julien.rouhaud@free.fr>
Date: Wed, 24 Mar 2021 16:27:40 +0800
Subject: [PATCH] Fix hash size estimation compatbility for PG11 and below.

Thanks to github user nikhil-postgres for the report.

While at it also fix a thinko in the size estimation which lead to greatly
overestimated hypothetical hash index sizes.
---
 CONTRIBUTORS.md |  1 +
 hypopg_index.c  | 20 +++++++++++++++++---
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/CONTRIBUTORS.md b/CONTRIBUTORS.md
index 1da12f9..7c96ed9 100644
--- a/CONTRIBUTORS.md
+++ b/CONTRIBUTORS.md
@@ -16,3 +16,4 @@ People who contributed to hypopg:
   * Extortioner01
   * nagaraju11
   * ibrahim edib kokdemir
+  * github user nikhil-postgres
diff --git a/hypopg_index.c b/hypopg_index.c
index 0680585..dbf445d 100644
--- a/hypopg_index.c
+++ b/hypopg_index.c
@@ -57,6 +57,9 @@
 #endif
 #include "parser/parse_utilcmd.h"
 #include "parser/parser.h"
+#if PG_VERSION_NUM >= 120000
+#include "port/pg_bitutils.h"
+#endif
 #include "storage/bufmgr.h"
 #include "utils/builtins.h"
 #include "utils/lsyscache.h"
@@ -1907,6 +1910,7 @@ hypo_estimate_index(hypoIndex * entry, RelOptInfo *rel)
 		uint32 num_buckets;
 		uint32 num_overflow;
 		uint32 num_bitmap;
+		uint32 lshift;
 
 	/*
 	 * Determine the target fill factor (in tuples per bucket) for this index.
@@ -1947,12 +1951,22 @@ hypo_estimate_index(hypoIndex * entry, RelOptInfo *rel)
 	num_overflow = Max(0, ((entry->tuples - (num_buckets * ffactor)) /
 					   ffactor) + 1);
 
+	/* find largest bitmap array size that will fit in page size */
+#if PG_VERSION_NUM >= 120000
+	lshift = pg_leftmost_one_pos32(HypoHashGetMaxBitmapSize());
+#else
+	for (lshift = _hash_log2(HypoHashGetMaxBitmapSize()); lshift > 0; --lshift)
+	{
+		if ((1 << lshift) <= HypoHashGetMaxBitmapSize())
+			break;
+	}
+#endif
+
 	/*
-	 * Naive estimate of bitmap pages, using the previously computed  number of
+	 * Naive estimate of bitmap pages, using the previously computed number of
 	 * overflow pages.
 	 */
-	num_bitmap = Max(1, num_overflow /
-					 pg_leftmost_one_pos32(HypoHashGetMaxBitmapSize()));
+	num_bitmap = Max(1, num_overflow / (1 <<lshift));
 
 	/* Simply add all computed pages, plus one extra block for the meta page */
 	entry->pages = num_buckets + num_overflow + num_bitmap + 1;