add show create table test for ctas

Signed-off-by: Bugen Zhao <[email protected]>

e2e_test/ddl/show.slt

@@ -1,6 +1,10 @@
 statement ok
 create table if not exists t3 (v1 int, v2 int, v3 int) append only;
 
+# Test definition purification for `CREATE TABLE AS`
+statement ok
+create table t3as as select v1 + 1 as vv1, v3 from t3;
+
 statement ok
 create materialized view mv3 as select sum(v1) as sum_v1 from t3;
 
@@ -97,11 +101,13 @@ rw_catalog
 query T
 show tables;
 ----
+t3as
 t3
 
 query T
 show tables from public;
 ----
+t3as
 t3
 
 query T
@@ -145,6 +151,11 @@ show create table t3;
 ----
 public.t3 CREATE TABLE t3 (v1 INT, v2 INT, v3 INT) APPEND ONLY
 
+query TT
+show create table t3as;
+----
+public.t3as	CREATE TABLE t3as (vv1 INT, v3 INT)
+
 query TT
 show create materialized view mv3;
 ----
@@ -196,6 +207,9 @@ drop sink sink3;
 statement ok
 drop table t3;
 
+statement ok
+drop table t3as;
+
 query TTTT
 describe pg_matviews;
 ----