Don't assume that max offset number stays fixed on a page when we're

not holding a pin on the page.  Use double instead of long to count
rows in relation, so that code still works for > LONG_MAX rows in rel.

Author: tglsfdc

src/backend/commands/analyze.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/commands/analyze.c,v 1.21 2001/06/22 19:16:21 wieck Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/commands/analyze.c,v 1.22 2001/07/05 19:33:35 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -97,8 +97,8 @@ typedef struct
 } ScalarMCVItem;
 
 
-#define swapInt(a,b)	{int _tmp; _tmp=a; a=b; b=_tmp;}
-#define swapDatum(a,b)	{Datum _tmp; _tmp=a; a=b; b=_tmp;}
+#define swapInt(a,b)	do {int _tmp; _tmp=a; a=b; b=_tmp;} while(0)
+#define swapDatum(a,b)	do {Datum _tmp; _tmp=a; a=b; b=_tmp;} while(0)
 
 
 static int MESSAGE_LEVEL;
@@ -111,20 +111,18 @@ static int *datumCmpTupnoLink;
 
 static VacAttrStats *examine_attribute(Relation onerel, int attnum);
 static int acquire_sample_rows(Relation onerel, HeapTuple *rows,
-							   int targrows, long *totalrows);
+							   int targrows, double *totalrows);
 static double random_fract(void);
 static double init_selection_state(int n);
-static long select_next_random_record(long t, int n, double *stateptr);
+static double select_next_random_record(double t, int n, double *stateptr);
 static int compare_rows(const void *a, const void *b);
 static int compare_scalars(const void *a, const void *b);
 static int compare_mcvs(const void *a, const void *b);
-static OffsetNumber get_page_max_offset(Relation relation,
-										BlockNumber blocknumber);
 static void compute_minimal_stats(VacAttrStats *stats,
-								  TupleDesc tupDesc, long totalrows,
+								  TupleDesc tupDesc, double totalrows,
 								  HeapTuple *rows, int numrows);
 static void compute_scalar_stats(VacAttrStats *stats,
-								 TupleDesc tupDesc, long totalrows,
+								 TupleDesc tupDesc, double totalrows,
 								 HeapTuple *rows, int numrows);
 static void update_attstats(Oid relid, int natts, VacAttrStats **vacattrstats);
 
@@ -143,7 +141,7 @@ analyze_rel(Oid relid, VacuumStmt *vacstmt)
 	VacAttrStats **vacattrstats;
 	int			targrows,
 				numrows;
-	long		totalrows;
+	double		totalrows;
 	HeapTuple  *rows;
 	HeapTuple	tuple;
 
@@ -298,7 +296,7 @@ analyze_rel(Oid relid, VacuumStmt *vacstmt)
 	if (!vacstmt->vacuum)
 		vac_update_relstats(RelationGetRelid(onerel),
 							onerel->rd_nblocks,
-							(double) totalrows,
+							totalrows,
 							RelationGetForm(onerel)->relhasindex);
 
 	/*
@@ -488,7 +486,7 @@ examine_attribute(Relation onerel, int attnum)
  */
 static int
 acquire_sample_rows(Relation onerel, HeapTuple *rows, int targrows,
-					long *totalrows)
+					double *totalrows)
 {
 	int			numrows = 0;
 	HeapScanDesc scan;
@@ -499,7 +497,7 @@ acquire_sample_rows(Relation onerel, HeapTuple *rows, int targrows,
 	OffsetNumber lastoffset;
 	int			numest;
 	double		tuplesperpage;
-	long		t;
+	double		t;
 	double		rstate;
 
 	Assert(targrows > 1);
@@ -520,7 +518,7 @@ acquire_sample_rows(Relation onerel, HeapTuple *rows, int targrows,
 	 */
 	if (!HeapTupleIsValid(tuple))
 	{
-		*totalrows = numrows;
+		*totalrows = (double) numrows;
 		return numrows;
 	}
 	/*
@@ -565,20 +563,22 @@ acquire_sample_rows(Relation onerel, HeapTuple *rows, int targrows,
 	}
 	tuplesperpage = (double) numest / (double) estblock;
 
-	t = numrows;				/* t is the # of records processed so far */
+	t = (double) numrows;		/* t is the # of records processed so far */
 	rstate = init_selection_state(targrows);
 	for (;;)
 	{
 		double			targpos;
 		BlockNumber		targblock;
+		Buffer			targbuffer;
+		Page			targpage;
 		OffsetNumber	targoffset,
 						maxoffset;
 
 		t = select_next_random_record(t, targrows, &rstate);
 		/* Try to read the t'th record in the table */
-		targpos = (double) t / tuplesperpage;
+		targpos = t / tuplesperpage;
 		targblock = (BlockNumber) targpos;
-		targoffset = ((int) (targpos - targblock) * tuplesperpage) + 
+		targoffset = ((int) ((targpos - targblock) * tuplesperpage)) + 
 			FirstOffsetNumber;
 		/* Make sure we are past the last selected record */
 		if (targblock <= lastblock)
@@ -595,21 +595,37 @@ acquire_sample_rows(Relation onerel, HeapTuple *rows, int targrows,
 		 */
 		if (targblock >= onerel->rd_nblocks)
 			break;
-		maxoffset = get_page_max_offset(onerel, targblock);
+		/*
+		 * We must maintain a pin on the target page's buffer to ensure that
+		 * the maxoffset value stays good (else concurrent VACUUM might
+		 * delete tuples out from under us).  Hence, pin the page until we
+		 * are done looking at it.  We don't maintain a lock on the page,
+		 * so tuples could get added to it, but we ignore such tuples.
+		 */
+		targbuffer = ReadBuffer(onerel, targblock);
+		if (!BufferIsValid(targbuffer))
+			elog(ERROR, "acquire_sample_rows: ReadBuffer(%s,%u) failed",
+				 RelationGetRelationName(onerel), targblock);
+		LockBuffer(targbuffer, BUFFER_LOCK_SHARE);
+		targpage = BufferGetPage(targbuffer);
+		maxoffset = PageGetMaxOffsetNumber(targpage);
+		LockBuffer(targbuffer, BUFFER_LOCK_UNLOCK);
+
 		for (;;)
 		{
 			HeapTupleData targtuple;
-			Buffer		targbuffer;
+			Buffer		tupbuffer;
 
 			if (targoffset > maxoffset)
 			{
 				/* Fell off end of this page, try next */
+				ReleaseBuffer(targbuffer);
 				targblock++;
 				targoffset = FirstOffsetNumber;
 				goto pageloop;
 			}
 			ItemPointerSet(&targtuple.t_self, targblock, targoffset);
-			heap_fetch(onerel, SnapshotNow, &targtuple, &targbuffer, NULL);
+			heap_fetch(onerel, SnapshotNow, &targtuple, &tupbuffer, NULL);
 			if (targtuple.t_data != NULL)
 			{
 				/*
@@ -621,6 +637,9 @@ acquire_sample_rows(Relation onerel, HeapTuple *rows, int targrows,
 				Assert(k >= 0 && k < targrows);
 				heap_freetuple(rows[k]);
 				rows[k] = heap_copytuple(&targtuple);
+				/* this releases the second pin acquired by heap_fetch: */
+				ReleaseBuffer(tupbuffer);
+				/* this releases the initial pin: */
 				ReleaseBuffer(targbuffer);
 				lastblock = targblock;
 				lastoffset = targoffset;
@@ -639,7 +658,7 @@ acquire_sample_rows(Relation onerel, HeapTuple *rows, int targrows,
 	/*
 	 * Estimate total number of valid rows in relation.
 	 */
-	*totalrows = (long) (onerel->rd_nblocks * tuplesperpage + 0.5);
+	*totalrows = floor((double) onerel->rd_nblocks * tuplesperpage + 0.5);
 
 	return numrows;
 }
@@ -667,6 +686,12 @@ random_fract(void)
  * of the last record processed and next record to process.  The only extra
  * state needed between calls is W, a random state variable.
  *
+ * Note: the original algorithm defines t, S, numer, and denom as integers.
+ * Here we express them as doubles to avoid overflow if the number of rows
+ * in the table exceeds INT_MAX.  The algorithm should work as long as the
+ * row count does not become so large that it is not represented accurately
+ * in a double (on IEEE-math machines this would be around 2^52 rows).
+ *
  * init_selection_state computes the initial W value.
  *
  * Given that we've already processed t records (t >= n),
@@ -680,36 +705,36 @@ init_selection_state(int n)
 	return exp(- log(random_fract())/n);
 }
 
-static long
-select_next_random_record(long t, int n, double *stateptr)
+static double
+select_next_random_record(double t, int n, double *stateptr)
 {
 	/* The magic constant here is T from Vitter's paper */
-	if (t <= (22 * n))
+	if (t <= (22.0 * n))
 	{
 		/* Process records using Algorithm X until t is large enough */
 		double	V,
 				quot;
 
 		V = random_fract();		/* Generate V */
-		t++;
-		quot = (double) (t - n) / (double) t;
+		t += 1;
+		quot = (t - (double) n) / t;
 		/* Find min S satisfying (4.1) */
 		while (quot > V)
 		{
-			t++;
-			quot *= (double) (t - n) / (double) t;
+			t += 1;
+			quot *= (t - (double) n) / t;
 		}
 	}
 	else
 	{
 		/* Now apply Algorithm Z */
 		double	W = *stateptr;
-		long	term = t - n + 1;
-		int		S;
+		double	term = t - (double) n + 1;
+		double	S;
 
 		for (;;)
 		{
-			long	numer,
+			double	numer,
 					numer_lim,
 					denom;
 			double	U,
@@ -722,9 +747,9 @@ select_next_random_record(long t, int n, double *stateptr)
 			/* Generate U and X */
 			U = random_fract();
 			X = t * (W - 1.0);
-			S = X;				/* S is tentatively set to floor(X) */
+			S = floor(X);		/* S is tentatively set to floor(X) */
 			/* Test if U <= h(S)/cg(X) in the manner of (6.3) */
-			tmp = (double) (t + 1) / (double) term;
+			tmp = (t + 1) / term;
 			lhs = exp(log(((U * tmp * tmp) * (term + S))/(t + X))/n);
 			rhs = (((t + X)/(term + S)) * term)/t;
 			if (lhs <= rhs)
@@ -734,20 +759,20 @@ select_next_random_record(long t, int n, double *stateptr)
 			}
 			/* Test if U <= f(S)/cg(X) */
 			y = (((U * (t + 1))/term) * (t + S + 1))/(t + X);
-			if (n < S)
+			if ((double) n < S)
 			{
 				denom = t;
 				numer_lim = term + S;
 			}
 			else
 			{
-				denom = t - n + S;
+				denom = t - (double) n + S;
 				numer_lim = t + 1;
 			}
-			for (numer = t + S; numer >= numer_lim; numer--)
+			for (numer = t + S; numer >= numer_lim; numer -= 1)
 			{
-				y *= (double) numer / (double) denom;
-				denom--;
+				y *= numer / denom;
+				denom -= 1;
 			}
 			W = exp(- log(random_fract())/n); /* Generate W in advance */
 			if (exp(log(y)/n) <= (t + X)/t)
@@ -783,30 +808,6 @@ compare_rows(const void *a, const void *b)
 	return 0;
 }
 
-/*
- * Discover the largest valid tuple offset number on the given page
- *
- * This code probably ought to live in some other module.
- */
-static OffsetNumber
-get_page_max_offset(Relation relation, BlockNumber blocknumber)
-{
-	Buffer		buffer;
-	Page		p;
-	OffsetNumber offnum;
-
-	buffer = ReadBuffer(relation, blocknumber);
-	if (!BufferIsValid(buffer))
-		elog(ERROR, "get_page_max_offset: %s relation: ReadBuffer(%ld) failed",
-			 RelationGetRelationName(relation), (long) blocknumber);
-	LockBuffer(buffer, BUFFER_LOCK_SHARE);
-	p = BufferGetPage(buffer);
-	offnum = PageGetMaxOffsetNumber(p);
-	LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
-	ReleaseBuffer(buffer);
-	return offnum;
-}
-
 
 /*
  *	compute_minimal_stats() -- compute minimal column statistics
@@ -825,7 +826,7 @@ get_page_max_offset(Relation relation, BlockNumber blocknumber)
  */
 static void
 compute_minimal_stats(VacAttrStats *stats,
-					  TupleDesc tupDesc, long totalrows,
+					  TupleDesc tupDesc, double totalrows,
 					  HeapTuple *rows, int numrows)
 {
 	int			i;
@@ -1002,7 +1003,7 @@ compute_minimal_stats(VacAttrStats *stats,
 
 			if (f1 < 1)
 				f1 = 1;
-			term1 = sqrt((double) totalrows / (double) numrows) * f1;
+			term1 = sqrt(totalrows / (double) numrows) * f1;
 			stats->stadistinct = floor(term1 + nmultiple + 0.5);
 		}
 
@@ -1104,7 +1105,7 @@ compute_minimal_stats(VacAttrStats *stats,
  */
 static void
 compute_scalar_stats(VacAttrStats *stats,
-					 TupleDesc tupDesc, long totalrows,
+					 TupleDesc tupDesc, double totalrows,
 					 HeapTuple *rows, int numrows)
 {
 	int			i;
@@ -1298,7 +1299,7 @@ compute_scalar_stats(VacAttrStats *stats,
 
 			if (f1 < 1)
 				f1 = 1;
-			term1 = sqrt((double) totalrows / (double) numrows) * f1;
+			term1 = sqrt(totalrows / (double) numrows) * f1;
 			stats->stadistinct = floor(term1 + nmultiple + 0.5);
 		}