diff --git a/src/backend/access/tablesample/bernoulli.c b/src/backend/access/tablesample/bernoulli.c
index 97d6407..e0a1568 100644
--- a/src/backend/access/tablesample/bernoulli.c
+++ b/src/backend/access/tablesample/bernoulli.c
@@ -46,7 +46,7 @@ typedef struct
 } BernoulliSamplerData;
 
 
-static void bernoulli_samplescancost(PlannerInfo *root,
+static void bernoulli_samplescangetrelsize(PlannerInfo *root,
 						 RelOptInfo *baserel,
 						 List *paramexprs,
 						 BlockNumber *pages,
@@ -73,7 +73,7 @@ tsm_bernoulli_handler(PG_FUNCTION_ARGS)
 	tsm->parameterTypes = list_make1_oid(FLOAT4OID);
 	tsm->repeatable_across_queries = true;
 	tsm->repeatable_across_scans = true;
-	tsm->SampleScanCost = bernoulli_samplescancost;
+	tsm->SampleScanGetRelSize = bernoulli_samplescangetrelsize;
 	tsm->InitSampleScan = bernoulli_initsamplescan;
 	tsm->BeginSampleScan = bernoulli_beginsamplescan;
 	tsm->NextSampleBlock = NULL;
@@ -87,11 +87,11 @@ tsm_bernoulli_handler(PG_FUNCTION_ARGS)
  * Cost estimation.
  */
 static void
-bernoulli_samplescancost(PlannerInfo *root,
-						 RelOptInfo *baserel,
-						 List *paramexprs,
-						 BlockNumber *pages,
-						 double *tuples)
+bernoulli_samplescangetrelsize(PlannerInfo *root,
+							   RelOptInfo *baserel,
+							   List *paramexprs,
+							   BlockNumber *pages,
+							   double *tuples)
 {
 	Node	   *pctnode;
 	float4		samplesize;
diff --git a/src/backend/access/tablesample/system.c b/src/backend/access/tablesample/system.c
index 7cece29..1cdb859 100644
--- a/src/backend/access/tablesample/system.c
+++ b/src/backend/access/tablesample/system.c
@@ -48,7 +48,7 @@ typedef struct
 } SystemSamplerData;
 
 
-static void system_samplescancost(PlannerInfo *root,
+static void system_samplescangetrelsize(PlannerInfo *root,
 					  RelOptInfo *baserel,
 					  List *paramexprs,
 					  BlockNumber *pages,
@@ -76,7 +76,7 @@ tsm_system_handler(PG_FUNCTION_ARGS)
 	tsm->parameterTypes = list_make1_oid(FLOAT4OID);
 	tsm->repeatable_across_queries = true;
 	tsm->repeatable_across_scans = true;
-	tsm->SampleScanCost = system_samplescancost;
+	tsm->SampleScanGetRelSize = system_samplescangetrelsize;
 	tsm->InitSampleScan = system_initsamplescan;
 	tsm->BeginSampleScan = system_beginsamplescan;
 	tsm->NextSampleBlock = system_nextsampleblock;
@@ -90,11 +90,11 @@ tsm_system_handler(PG_FUNCTION_ARGS)
  * Cost estimation.
  */
 static void
-system_samplescancost(PlannerInfo *root,
-					  RelOptInfo *baserel,
-					  List *paramexprs,
-					  BlockNumber *pages,
-					  double *tuples)
+system_samplescangetrelsize(PlannerInfo *root,
+							RelOptInfo *baserel,
+							List *paramexprs,
+							BlockNumber *pages,
+							double *tuples)
 {
 	Node	   *pctnode;
 	float4		samplesize;
diff --git a/src/backend/executor/nodeSamplescan.c b/src/backend/executor/nodeSamplescan.c
index 32db05e..144d2db 100644
--- a/src/backend/executor/nodeSamplescan.c
+++ b/src/backend/executor/nodeSamplescan.c
@@ -325,6 +325,12 @@ tablesample_init(SampleScanState *scanstate)
 		 * suitable seed.  For regression-testing purposes, that has the
 		 * convenient property that REPEATABLE(0) gives a machine-independent
 		 * result.
+		 *
+		 * The reson for using float8 for REPEATABLE at the SQL level is that
+		 * it will work as expected both for users used to databases which
+		 * accept only integers in REPEATABLE clause and for those who might
+		 * expect that REPEATABLE works same way as setseed() (float in the
+		 * range from -1 to 1).
 		 */
 		seed = DatumGetUInt32(DirectFunctionCall1(hashfloat8, datum));
 	}
diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index 187db96..5c114f4 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -503,8 +503,8 @@ set_tablesample_rel_size(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
 	 * we assume the function returns sane values.)
 	 */
 	tsm = GetTsmRoutine(tsc->tsmhandler);
-	tsm->SampleScanCost(root, rel, tsc->args,
-						&pages, &tuples);
+	tsm->SampleScanGetRelSize(root, rel, tsc->args,
+							  &pages, &tuples);
 
 	/*
 	 * If the sampling method does not support repeatable scans, we cannot
diff --git a/src/include/access/tsmapi.h b/src/include/access/tsmapi.h
index c3b0cab..90f2395 100644
--- a/src/include/access/tsmapi.h
+++ b/src/include/access/tsmapi.h
@@ -20,7 +20,7 @@
  * Callback function signatures --- see tablesample-method.sgml for more info.
  */
 
-typedef void (*SampleScanCost_function) (PlannerInfo *root,
+typedef void (*SampleScanGetRelSize_function) (PlannerInfo *root,
 													 RelOptInfo *baserel,
 													 List *paramexprs,
 													 BlockNumber *pages,
@@ -64,7 +64,7 @@ typedef struct TsmRoutine
 	bool		repeatable_across_scans;
 
 	/* Functions for planning a SampleScan on a physical table */
-	SampleScanCost_function SampleScanCost;
+	SampleScanGetRelSize_function SampleScanGetRelSize;
 
 	/* Functions for executing a SampleScan on a physical table */
 	InitSampleScan_function InitSampleScan;		/* can be NULL */
diff --git a/src/test/regress/expected/tablesample.out b/src/test/regress/expected/tablesample.out
index 23ea8cb..55f89bd 100644
--- a/src/test/regress/expected/tablesample.out
+++ b/src/test/regress/expected/tablesample.out
@@ -232,6 +232,8 @@ SELECT id FROM test_tablesample TABLESAMPLE FOOBAR (1);
 ERROR:  tablesample method foobar does not exist
 LINE 1: SELECT id FROM test_tablesample TABLESAMPLE FOOBAR (1);
                                                     ^
+SELECT id FROM test_tablesample TABLESAMPLE SYSTEM (NULL);
+ERROR:  TABLESAMPLE parameter cannot be null
 SELECT id FROM test_tablesample TABLESAMPLE SYSTEM (50) REPEATABLE (NULL);
 ERROR:  TABLESAMPLE REPEATABLE parameter cannot be null
 SELECT id FROM test_tablesample TABLESAMPLE BERNOULLI (-1);
diff --git a/src/test/regress/sql/tablesample.sql b/src/test/regress/sql/tablesample.sql
index e4b5636..9406a19 100644
--- a/src/test/regress/sql/tablesample.sql
+++ b/src/test/regress/sql/tablesample.sql
@@ -68,6 +68,7 @@ select pct, count(unique1) from
 -- errors
 SELECT id FROM test_tablesample TABLESAMPLE FOOBAR (1);
 
+SELECT id FROM test_tablesample TABLESAMPLE SYSTEM (NULL);
 SELECT id FROM test_tablesample TABLESAMPLE SYSTEM (50) REPEATABLE (NULL);
 
 SELECT id FROM test_tablesample TABLESAMPLE BERNOULLI (-1);