Make DISCARD SEQUENCES also discard the last used sequence.

Otherwise, we access already-freed memory.  Oops.

Report by Michael Paquier.  Fix by me.

3 files changed, 7 insertions, 0 deletions

diff --git a/src/backend/commands/sequence.c b/src/backend/commands/sequence.c
index eba7cad6ec3..f3344c6207d 100644
--- a/src/backend/commands/sequence.c
+++ b/src/backend/commands/sequence.c
@@ -1617,4 +1617,6 @@ ResetSequenceCaches(void)
 		free(seqtab);
 		seqtab = next;
 	}
+
+	last_used_seq = NULL;
 }
diff --git a/src/test/regress/expected/sequence.out b/src/test/regress/expected/sequence.out
index 37e9a8cfa1a..8fcb70015fb 100644
--- a/src/test/regress/expected/sequence.out
+++ b/src/test/regress/expected/sequence.out
@@ -344,6 +344,9 @@ SELECT lastval();
       99
 (1 row)
 
+DISCARD SEQUENCES;
+SELECT lastval();
+ERROR:  lastval is not yet defined in this session
 CREATE SEQUENCE seq2;
 SELECT nextval('seq2');
  nextval 
diff --git a/src/test/regress/sql/sequence.sql b/src/test/regress/sql/sequence.sql
index 31cbc926c02..be5e9a97bf4 100644
--- a/src/test/regress/sql/sequence.sql
+++ b/src/test/regress/sql/sequence.sql
@@ -154,6 +154,8 @@ SELECT nextval('seq');
 SELECT lastval();
 SELECT setval('seq', 99);
 SELECT lastval();
+DISCARD SEQUENCES;
+SELECT lastval();
 
 CREATE SEQUENCE seq2;
 SELECT nextval('seq2');