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commit    e805735a83f7bb5bfe773b9690f96a52ba2b84b2
Author:   Tom Lane <tgl@sss.pgh.pa.us>
Date:     Mon Apr 18 16:16:45 2022 +0000

    Avoid invalid array reference in transformAlterTableStmt().
    
    Don't try to look at the attidentity field of system attributes,
    because they're not there in the TupleDescAttr array.  Sometimes
    this is harmless because we accidentally pick up a zero, but
    otherwise we'll report "no owned sequence found" from an attempt
    to alter a system attribute.  (It seems possible that a SIGSEGV
    could occur, too, though I've not seen it in testing.)
    
    It's not in this function's charter to complain that you can't
    alter a system column, so instead just hard-wire an assumption
    that system attributes aren't identities.  I didn't bother with
    a regression test because the appearance of the bug is very
    erratic.
    
    Per bug #17465 from Roman Zharkov.  Back-patch to all supported
    branches.  (There's not actually a live bug before v12, because
    before that get_attidentity() did the right thing anyway.
    But for consistency I changed the test in the older branches too.)
    
    Discussion: https://postgr.es/m/17465-f2a554a6cb5740d3@postgresql.org


 src/backend/parser/parse_utilcmd.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/backend/parser/parse_utilcmd.c b/src/backend/parser/parse_utilcmd.c
index c1d5a5d540b..05e4fa8ce8c 100644
--- a/src/backend/parser/parse_utilcmd.c
+++ b/src/backend/parser/parse_utilcmd.c
@@ -3421,7 +3421,8 @@ transformAlterTableStmt(Oid relid, AlterTableStmt *stmt,
 								 errmsg("column \"%s\" of relation \"%s\" does not exist",
 										cmd->name, RelationGetRelationName(rel))));
 
-					if (TupleDescAttr(tupdesc, attnum - 1)->attidentity)
+					if (attnum > 0 &&
+						TupleDescAttr(tupdesc, attnum - 1)->attidentity)
 					{
 						Oid			seq_relid = getIdentitySequence(relid, attnum, false);
 						Oid			typeOid = typenameTypeId(pstate, def->typeName);


[parent: 8bcf90c7a67c]