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commit    36d4efe779bfc7190ea1c1cf8deb0d945b726663
Author:   Tom Lane <tgl@sss.pgh.pa.us>
Date:     Mon Apr 18 16:16:45 2022 +0000
First released in: 15.0, 10.21, 11.16, 12.11, 13.7, 14.3

    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 2826559d09b..1a64a522798 100644
--- a/src/backend/parser/parse_utilcmd.c
+++ b/src/backend/parser/parse_utilcmd.c
@@ -3424,7 +3424,8 @@ 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: 1a8b110539ef]