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diff --git a/src/test/isolation/specs/horizons.spec b/src/test/isolation/specs/horizons.spec
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+# Test that pruning and vacuuming pay attention to concurrent sessions
+# in the right way. For normal relations that means that rows cannot
+# be pruned away if there's an older snapshot, in contrast to that
+# temporary tables should nearly always be prunable.
+#
+# NB: Think hard before adding a test showing that rows in permanent
+# tables get pruned - it's quite likely that it'd be racy, e.g. due to
+# an autovacuum worker holding a snapshot.
+
+setup {
+    CREATE OR REPLACE FUNCTION explain_json(p_query text)
+    RETURNS json
+    LANGUAGE plpgsql AS $$
+        DECLARE
+            v_ret json;
+        BEGIN
+            EXECUTE p_query INTO STRICT v_ret;
+            RETURN v_ret;
+        END;$$;
+}
+
+teardown {
+    DROP FUNCTION explain_json(text);
+}
+
+session lifeline
+
+# Start a transaction, force a snapshot to be held
+step ll_start
+{
+    BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
+    SELECT 1;
+}
+
+step ll_commit { COMMIT; }
+
+
+session pruner
+
+setup
+{
+    SET enable_seqscan = false;
+    SET enable_indexscan = false;
+    SET enable_bitmapscan = false;
+}
+
+step pruner_create_temp
+{
+    CREATE TEMPORARY TABLE horizons_tst (data int unique) WITH (autovacuum_enabled = off);
+    INSERT INTO horizons_tst(data) VALUES(1),(2);
+}
+
+step pruner_create_perm
+{
+    CREATE TABLE horizons_tst (data int unique) WITH (autovacuum_enabled = off);
+    INSERT INTO horizons_tst(data) VALUES(1),(2);
+}
+
+# Temp tables cannot be dropped in the teardown, so just always do so
+# as part of the permutation
+step pruner_drop
+{
+    DROP TABLE horizons_tst;
+}
+
+step pruner_delete
+{
+    DELETE FROM horizons_tst;
+}
+
+step pruner_begin { BEGIN; }
+step pruner_commit { COMMIT; }
+
+step pruner_vacuum
+{
+    VACUUM horizons_tst;
+}
+
+# Show the heap fetches of an ordered index-only-scan (other plans
+# have been forbidden above) - that tells us how many non-killed leaf
+# entries there are.
+step pruner_query
+{
+    SELECT explain_json($$
+        EXPLAIN (FORMAT json, BUFFERS, ANALYZE)
+	  SELECT * FROM horizons_tst ORDER BY data;$$)->0->'Plan'->'Heap Fetches';
+}
+
+# Verify that the query plan still is an IOS
+step pruner_query_plan
+{
+    EXPLAIN (COSTS OFF) SELECT * FROM horizons_tst ORDER BY data;
+}
+
+
+# Show that with a permanent relation deleted rows cannot be pruned
+# away if there's a concurrent session still seeing the rows.
+permutation
+    pruner_create_perm
+    ll_start
+    pruner_query_plan
+    # Run query that could do pruning twice, first has chance to prune,
+    # second would not perform heap fetches if first query did.
+    pruner_query
+    pruner_query
+    pruner_delete
+    pruner_query
+    pruner_query
+    ll_commit
+    pruner_drop
+
+# Show that with a temporary relation deleted rows can be pruned away,
+# even if there's a concurrent session with a snapshot from before the
+# deletion. That's safe because the session with the older snapshot
+# cannot access the temporary table.
+permutation
+    pruner_create_temp
+    ll_start
+    pruner_query_plan
+    pruner_query
+    pruner_query
+    pruner_delete
+    pruner_query
+    pruner_query
+    ll_commit
+    pruner_drop
+
+# Verify that pruning in temporary relations doesn't remove rows still
+# visible in the current session
+permutation
+    pruner_create_temp
+    ll_start
+    pruner_query
+    pruner_query
+    pruner_begin
+    pruner_delete
+    pruner_query
+    pruner_query
+    ll_commit
+    pruner_commit
+    pruner_drop
+
+# Show that vacuum cannot remove deleted rows still visible to another
+# session's snapshot, when accessing a permanent table.
+permutation
+    pruner_create_perm
+    ll_start
+    pruner_query
+    pruner_query
+    pruner_delete
+    pruner_vacuum
+    pruner_query
+    pruner_query
+    ll_commit
+    pruner_drop
+
+# Show that vacuum can remove deleted rows still visible to another
+# session's snapshot, when accessing a temporary table.
+permutation
+    pruner_create_temp
+    ll_start
+    pruner_query
+    pruner_query
+    pruner_delete
+    pruner_vacuum
+    pruner_query
+    pruner_query
+    ll_commit
+    pruner_drop