Use standard librart sqrt function in pg_stat_statements

The stddev calculation included a faster but unportable sqrt function.
This is not worth the extra effort, and won't work everywhere. If the
standard library function is good enough for the SQL function it
should be good enough here too.

contrib/pg_stat_statements/pg_stat_statements.c

@@ -57,6 +57,7 @@
  */
 #include "postgres.h"
 
+#include <math.h>
 #include <sys/stat.h>
 #include <unistd.h>
 
@@ -326,7 +327,6 @@ static char *generate_normalized_query(pgssJumbleState *jstate, const char *quer
 						  int *query_len_p, int encoding);
 static void fill_in_constant_lengths(pgssJumbleState *jstate, const char *query);
 static int	comp_location(const void *a, const void *b);
-static inline double sqrtd(const double x);
 
 
 /*
@@ -1583,7 +1583,7 @@ pg_stat_statements_internal(FunctionCallInfo fcinfo,
 			 */
 			if (tmp.calls > 1)
 				values[i++] =
-					Float8GetDatum(sqrtd(tmp.sum_var_time / tmp.calls));
+					Float8GetDatum(sqrt(tmp.sum_var_time / tmp.calls));
 			else
 				values[i++] = Float8GetDatum(0.0);
 		}
@@ -2968,20 +2968,3 @@ comp_location(const void *a, const void *b)
 	else
 		return 0;
 }
-
-/*
- * fast sqrt algorithm: reference from Fast inverse square root algorithms.
- */
-static inline double
-sqrtd(const double x)
-{
-	double      x_half = 0.5 * x;
-	long long int   tmp = 0x5FE6EB50C7B537AAl - ( *(long long int*)&x >> 1);
-	double      x_result = * (double*)&tmp;
-
-	x_result *= (1.5 - (x_half * x_result * x_result));
-	/* If retry this calculation, it becomes higher precision at sqrt */
-	x_result *= (1.5 - (x_half * x_result * x_result));
-
-	return x_result * x;
-}