diff --git a/pgxn/neon/file_cache.c b/pgxn/neon/file_cache.c
index 8dff259f02..8670650ff4 100644
--- a/pgxn/neon/file_cache.c
+++ b/pgxn/neon/file_cache.c
@@ -17,6 +17,7 @@
 #include <sys/statvfs.h>
 #include <unistd.h>
 #include <fcntl.h>
+#include <math.h>
 
 #include "postgres.h"
 #include "funcapi.h"
@@ -66,6 +67,126 @@
 #define MAX_MONITOR_INTERVAL_USEC 1000000 /* 1 second */
 #define MAX_DISK_WRITE_RATE       1000 /* MB/sec */
 
+/* Hyper log-log implementation for counting size of working set */
+
+#define ROTL32(x, r) ((x) << (r)) | ((x) >> (32 - (r)))
+#define HASH_BITS 25
+#define N_HASHES (1 << (32 - HASH_BITS))
+#define MURMUR_SEED 0x5C1DB
+
+static uint32
+murmur_hash3_32(const void* key, const int len, const uint32 seed)
+{
+    const uint8 * data = (const uint8*)key;
+    const int nblocks = len / 4;
+
+    uint32 h1 = seed;
+
+    uint32 c1 = 0xcc9e2d51;
+    uint32 c2 = 0x1b873593;
+    int i;
+    uint32 k1;
+    const uint8* tail;
+    const uint32* blocks = (const uint32 *)(data + nblocks*4);
+
+    for(i = -nblocks; i; i++)
+    {
+        k1 = blocks[i];
+
+        k1 *= c1;
+        k1 = ROTL32(k1,15);
+        k1 *= c2;
+
+        h1 ^= k1;
+        h1 = ROTL32(h1,13);
+        h1 = h1*5+0xe6546b64;
+    }
+
+    tail = (const uint8*)(data + nblocks*4);
+
+    k1 = 0;
+
+    switch(len & 3)
+    {
+      case 3:
+        k1 ^= tail[2] << 16;
+        /* no break */
+      case 2:
+        k1 ^= tail[1] << 8;
+        /* no break */
+      case 1:
+        k1 ^= tail[0];
+        k1 *= c1;
+        k1 = ROTL32(k1,15);
+        k1 *= c2;
+        h1 ^= k1;
+    }
+
+    h1 ^= len;
+    h1 ^= h1 >> 16;
+    h1 *= 0x85ebca6b;
+    h1 ^= h1 >> 13;
+    h1 *= 0xc2b2ae35;
+    h1 ^= h1 >> 16;
+
+    return h1;
+}
+
+static void
+calculate_zero_bits(uint32 h, uint8* max_zero_bits)
+{
+    int j = h >> HASH_BITS;
+    int zero_bits = 1;
+    while ((h & 1) == 0 && zero_bits <= HASH_BITS)  {
+        h >>= 1;
+        zero_bits += 1;
+    }
+    if (max_zero_bits[j] < zero_bits) {
+        max_zero_bits[j] = zero_bits;
+    }
+}
+
+static void
+calculate_hash_functions(void const* val, int size, uint8* max_zero_bits)
+{
+    uint32 h = murmur_hash3_32(val, size, MURMUR_SEED);
+    calculate_zero_bits(h, max_zero_bits);
+}
+
+static uint32
+approximate_distinct_count(uint8* max_zero_bits)
+{
+    const int m = N_HASHES;
+    const double alpha_m = 0.7213 / (1 + 1.079 / (double)m);
+    const double pow_2_32 = 0xffffffff;
+    double E, c = 0;
+    int i;
+    for (i = 0; i < m; i++)
+    {
+        c += 1 / pow(2., (double)max_zero_bits[i]);
+    }
+    E = alpha_m * m * m / c;
+
+    if (E <= (5 / 2. * m))
+    {
+        double V = 0;
+        for (i = 0; i < m; i++)
+        {
+            if (max_zero_bits[i] == 0) V++;
+        }
+
+        if (V > 0)
+        {
+            E = m * log(m / V);
+        }
+    }
+    else if (E > (1 / 30. * pow_2_32))
+    {
+        E = -pow_2_32 * log(1 - E / pow_2_32);
+    }
+    return (uint32)E;
+}
+
 typedef struct FileCacheEntry
 {
 	BufferTag	key;
@@ -77,9 +198,10 @@ typedef struct FileCacheEntry
 
 typedef struct FileCacheControl
 {
-	uint32 size; /* size of cache file in chunks */
-	uint32 used; /* number of used chunks */
-	dlist_head lru; /* double linked list for LRU replacement algorithm */
+	uint32		size; /* size of cache file in chunks */
+	uint32		used; /* number of used chunks */
+	dlist_head	lru; /* double linked list for LRU replacement algorithm */
+	uint8		max_zero_bits[N_HASHES];
 } FileCacheControl;
 
 static HTAB* lfc_hash;
@@ -124,6 +246,7 @@ lfc_shmem_startup(void)
 		lfc_ctl->size = 0;
 		lfc_ctl->used = 0;
 		dlist_init(&lfc_ctl->lru);
+		memset(lfc_ctl->max_zero_bits, 0, sizeof lfc_ctl->max_zero_bits);
 
 		/* Remove file cache on restart */
 		(void)unlink(lfc_path);
@@ -395,7 +518,13 @@ lfc_read(RelFileNode rnode, ForkNumber forkNum, BlockNumber blkno,
 	hash = get_hash_value(lfc_hash, &tag);
 
 	LWLockAcquire(lfc_lock, LW_EXCLUSIVE);
+	calculate_hash_functions(&tag, sizeof(tag), lfc_ctl->max_zero_bits);
 	entry = hash_search_with_hash_value(lfc_hash, &tag, hash, HASH_FIND, NULL);
+
+	/* Approximate working set */
+	tag.blockNum = blkno;
+	calculate_hash_functions(&tag, sizeof(tag), lfc_ctl->max_zero_bits);
+
 	if (entry == NULL || (entry->bitmap[chunk_offs >> 5] & (1 << (chunk_offs & 31))) == 0)
 	{
 		/* Page is not cached */
@@ -698,3 +827,21 @@ local_cache_pages(PG_FUNCTION_ARGS)
 	else
 		SRF_RETURN_DONE(funcctx);
 }
+
+PG_FUNCTION_INFO_V1(approximate_working_set_size);
+
+Datum
+approximate_working_set_size(PG_FUNCTION_ARGS)
+{
+	int32 dc = -1;
+	if (lfc_size_limit != 0)
+	{
+		bool reset = PG_GETARG_BOOL(0);
+		LWLockAcquire(lfc_lock, reset ? LW_EXCLUSIVE : LW_SHARED);
+		dc = (int32)approximate_distinct_count(lfc_ctl->max_zero_bits);
+		if (reset)
+			memset(lfc_ctl->max_zero_bits, 0, sizeof lfc_ctl->max_zero_bits);
+		LWLockRelease(lfc_lock);
+	}
+	PG_RETURN_INT32(dc);
+}
diff --git a/pgxn/neon/neon--1.0.sql b/pgxn/neon/neon--1.0.sql
index 6cf111ea6a..78982b46cd 100644
--- a/pgxn/neon/neon--1.0.sql
+++ b/pgxn/neon/neon--1.0.sql
@@ -27,6 +27,11 @@ RETURNS SETOF RECORD
 AS 'MODULE_PATHNAME', 'local_cache_pages'
 LANGUAGE C PARALLEL SAFE;
 
+CREATE FUNCTION approximate_working_set_size(reset bool)
+RETURNS integer
+AS 'MODULE_PATHNAME', 'approximate_working_set_size'
+LANGUAGE C PARALLEL SAFE;
+
 -- Create a view for convenient access.
 CREATE VIEW local_cache AS
 	SELECT P.* FROM local_cache_pages() AS P