Redefine pg_class.reltuples to be -1 before the first VACUUM or ANALYZE.

Historically, we've considered the state with relpages and reltuples
both zero as indicating that we do not know the table's tuple density.
This is problematic because it's impossible to distinguish "never yet
vacuumed" from "vacuumed and seen to be empty".  In particular, a user
cannot use VACUUM or ANALYZE to override the planner's normal heuristic
that an empty table should not be believed to be empty because it is
probably about to get populated.  That heuristic is a good safety
measure, so I don't care to abandon it, but there should be a way to
override it if the table is indeed intended to stay empty.

Hence, represent the initial state of ignorance by setting reltuples
to -1 (relpages is still set to zero), and apply the minimum-ten-pages
heuristic only when reltuples is still -1.  If the table is empty,
VACUUM or ANALYZE (but not CREATE INDEX) will override that to
reltuples = relpages = 0, and then we'll plan on that basis.

This requires a bunch of fiddly little changes, but we can get rid of
some ugly kluges that were formerly needed to maintain the old definition.

One notable point is that FDWs' GetForeignRelSize methods will see
baserel->tuples = -1 when no ANALYZE has been done on the foreign table.
That seems like a net improvement, since those methods were formerly
also in the dark about what baserel->tuples = 0 really meant.  Still,
it is an API change.

I bumped catversion because code predating this change would get confused
by seeing reltuples = -1.

Discussion: https://postgr.es/m/F02298E0-6EF4-49A1-BCB6-C484794D9ACC@thebuild.com

2 files changed, 9 insertions, 8 deletions

diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index 0eeff804bcf..b399592ff81 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -912,7 +912,11 @@ set_foreign_size(PlannerInfo *root, RelOptInfo *rel, RangeTblEntry *rte)
 	/* ... but do not let it set the rows estimate to zero */
 	rel->rows = clamp_row_est(rel->rows);
 
-	/* also, make sure rel->tuples is not insane relative to rel->rows */
+	/*
+	 * Also, make sure rel->tuples is not insane relative to rel->rows.
+	 * Notably, this ensures sanity if pg_class.reltuples contains -1 and the
+	 * FDW doesn't do anything to replace that.
+	 */
 	rel->tuples = Max(rel->tuples, rel->rows);
 }
 
diff --git a/src/backend/optimizer/util/plancat.c b/src/backend/optimizer/util/plancat.c
index 25545029d7a..f9d0d67aa75 100644
--- a/src/backend/optimizer/util/plancat.c
+++ b/src/backend/optimizer/util/plancat.c
@@ -974,11 +974,6 @@ estimate_rel_size(Relation rel, int32 *attr_widths,
 			/* it has storage, ok to call the smgr */
 			curpages = RelationGetNumberOfBlocks(rel);
 
-			/* coerce values in pg_class to more desirable types */
-			relpages = (BlockNumber) rel->rd_rel->relpages;
-			reltuples = (double) rel->rd_rel->reltuples;
-			relallvisible = (BlockNumber) rel->rd_rel->relallvisible;
-
 			/* report estimated # pages */
 			*pages = curpages;
 			/* quick exit if rel is clearly empty */
@@ -988,6 +983,7 @@ estimate_rel_size(Relation rel, int32 *attr_widths,
 				*allvisfrac = 0;
 				break;
 			}
+
 			/* coerce values in pg_class to more desirable types */
 			relpages = (BlockNumber) rel->rd_rel->relpages;
 			reltuples = (double) rel->rd_rel->reltuples;
@@ -1006,12 +1002,12 @@ estimate_rel_size(Relation rel, int32 *attr_widths,
 			}
 
 			/* estimate number of tuples from previous tuple density */
-			if (relpages > 0)
+			if (reltuples >= 0 && relpages > 0)
 				density = reltuples / (double) relpages;
 			else
 			{
 				/*
-				 * When we have no data because the relation was truncated,
+				 * If we have no data because the relation was never vacuumed,
 				 * estimate tuple width from attribute datatypes.  We assume
 				 * here that the pages are completely full, which is OK for
 				 * tables (since they've presumably not been VACUUMed yet) but
@@ -1059,6 +1055,7 @@ estimate_rel_size(Relation rel, int32 *attr_widths,
 			break;
 		case RELKIND_FOREIGN_TABLE:
 			/* Just use whatever's in pg_class */
+			/* Note that FDW must cope if reltuples is -1! */
 			*pages = rel->rd_rel->relpages;
 			*tuples = rel->rd_rel->reltuples;
 			*allvisfrac = 0;