bitset: remove harmful optimization from index:count()

Fixes #1896

src/box/memtx_bitset.cc

@@ -404,17 +404,19 @@ MemtxBitset::count(enum iterator_type type, const char *key,
 	struct bit_iterator bit_it;
 	size_t bit;
 	if (type == ITER_BITS_ANY_SET) {
-		/*
-		 * Optimization: get the number of items for each requested bit
-		 * and then found the maximum.
+		/**
+		 * Optimization: for an empty key return 0.
 		 */
 		bit_iterator_init(&bit_it, bitset_key, bitset_key_size, true);
-		size_t result = 0;
-		while ((bit = bit_iterator_next(&bit_it)) != SIZE_MAX) {
-			size_t count = bitset_index_count(&m_index, bit);
-			result = MAX(result, count);
-		}
-		return result;
+		bit = bit_iterator_next(&bit_it);
+		if (bit == SIZE_MAX)
+			return 0;
+		/**
+		 * Optimiation: for a single bit key use
+		 * bitset_index_count().
+		 */
+		if (bit_iterator_next(&bit_it) == SIZE_MAX)
+			return bitset_index_count(&m_index, bit);
 	} else if (type == ITER_BITS_ALL_SET) {
 		/**
 		 * Optimization: for an empty key return the number of items

test/box/bitset.result

@@ -1880,3 +1880,63 @@ space:drop()
 space = nil
 ---
 ...
+-- https://github.com/tarantool/tarantool/issues/1896 wrong countspace = box.schema.space.create('test')
+s = box.schema.space.create('test')
+---
+...
+_ = s:create_index('primary', { type = 'hash', parts = {1, 'num'}, unique = true })
+---
+...
+i = s:create_index('bitset', { type = 'bitset', parts = {2, 'num'}, unique = false })
+---
+...
+s:insert{1, 0}
+---
+- [1, 0]
+...
+s:insert{2, 0}
+---
+- [2, 0]
+...
+s:insert{3, 0}
+---
+- [3, 0]
+...
+s:insert{4, 2}
+---
+- [4, 2]
+...
+s:insert{5, 2}
+---
+- [5, 2]
+...
+s:insert{6, 3}
+---
+- [6, 3]
+...
+s:insert{7, 4}
+---
+- [7, 4]
+...
+s:insert{8, 5}
+---
+- [8, 5]
+...
+s:insert{9, 8}
+---
+- [9, 8]
+...
+#i:select(7, {iterator = box.index.BITS_ANY_SET})
+---
+- 5
+...
+i:count(7, {iterator = box.index.BITS_ANY_SET})
+---
+- 5
+...
+s:drop()
+---
+...
+s = nil
+---
+...

test/box/bitset.test.lua

@@ -115,3 +115,20 @@ _ = space:create_index('bitset', { type = 'bitset', parts = {2, 'number'}, uniqu
 space:drop()
 space = nil
 
+-- https://github.com/tarantool/tarantool/issues/1896 wrong countspace = box.schema.space.create('test')
+s = box.schema.space.create('test')
+_ = s:create_index('primary', { type = 'hash', parts = {1, 'num'}, unique = true })
+i = s:create_index('bitset', { type = 'bitset', parts = {2, 'num'}, unique = false })
+s:insert{1, 0}
+s:insert{2, 0}
+s:insert{3, 0}
+s:insert{4, 2}
+s:insert{5, 2}
+s:insert{6, 3}
+s:insert{7, 4}
+s:insert{8, 5}
+s:insert{9, 8}
+#i:select(7, {iterator = box.index.BITS_ANY_SET})
+i:count(7, {iterator = box.index.BITS_ANY_SET})
+s:drop()
+s = nil