pg_dump: Adjust reltuples from 0 to -1 for dumps on older versions.

Before v14, a reltuples value of 0 was ambiguous: it could either
mean the relation is empty, or it could mean that it hadn't yet
been vacuumed or analyzed.  (Commit 3d351d9 taught v14 and newer
to use -1 for the latter case.)  This ambiguity can cause the
planner to choose inefficient plans after restoring to v18 or
newer.  To fix, let's just dump reltuples as -1 in that case.  This
will cause some truly empty tables to be seen as not-yet-processed,
but that seems unlikely to cause too much trouble in practice.

Commit 9879105 fixed a similar problem for vacuumdb by removing
the check for reltuples != 0.  Presumably we could reinstate that
check now, but I've chosen to leave it in place in case reltuples
isn't accurate.  As before, processing some empty tables seems
pretty innocuous.

Author: Hari Krishna Sunder <[email protected]>
Discussion: https://postgr.es/m/CAAeiqZ0o2p4SX5_xPcuAbbsmXjg6MJLNuPYSLUjC%3DWh-VeW64A%40mail.gmail.com

Author: nathan-bossart

src/bin/pg_dump/pg_dump.c

@@ -10929,7 +10929,20 @@ dumpRelationStats_dumper(Archive *fout, const void *userArg, const TocEntry *te)
 	appendStringLiteralAH(out, rsinfo->dobj.name, fout);
 	appendPQExpBufferStr(out, ",\n");
 	appendPQExpBuffer(out, "\t'relpages', '%d'::integer,\n", rsinfo->relpages);
-	appendPQExpBuffer(out, "\t'reltuples', '%s'::real,\n", rsinfo->reltuples);
+
+	/*
+	 * Before v14, a reltuples value of 0 was ambiguous: it could either mean
+	 * the relation is empty, or it could mean that it hadn't yet been
+	 * vacuumed or analyzed.  (Newer versions use -1 for the latter case.)
+	 * This ambiguity can cause the planner to choose inefficient plans after
+	 * restoring to v18 or newer.  To deal with this, let's just set reltuples
+	 * to -1 in that case.
+	 */
+	if (fout->remoteVersion < 140000 && strcmp("0", rsinfo->reltuples) == 0)
+		appendPQExpBufferStr(out, "\t'reltuples', '-1'::real,\n");
+	else
+		appendPQExpBuffer(out, "\t'reltuples', '%s'::real,\n", rsinfo->reltuples);
+
 	appendPQExpBuffer(out, "\t'relallvisible', '%d'::integer",
 					  rsinfo->relallvisible);