Re: Early WIP/PoC for inlining CTEs - Mailing list pgsql-hackers

I wrote:
> Therefore, I'm reversing my previous opinion that we should not have
> an explicit NOT MATERIALIZED option.  I think we should add that, and
> the behavior ought to be:
> * No option given: inline if there's exactly one reference.
> * With MATERIALIZED: never inline.
> * With NOT MATERIALIZED: inline regardless of the number of references.
> (Obviously, we should not inline if there's RECURSIVE or the CTE
> potentially has side-effects, regardless of the user option;
> I don't think those cases are up for debate.)

Hearing no immediate pushback on that proposal, I went ahead and made
a version of the patch that does it like that, as attached.  I also took
a stab at documenting it fully.

I was interested to find, while writing the docs, that it's a real
struggle to invent plausible reasons to write MATERIALIZED given the
above specification.  You pretty much have to have lied to the planner,
eg by making a volatile function that's not marked volatile, before
there's a real need for that.  Am I missing something?  If I'm not,
then we're in a really good place backwards-compatibility-wise,
because the new default behavior shouldn't break any cases where people
weren't cheating.

            regards, tom lane

diff --git a/contrib/pg_stat_statements/pg_stat_statements.c b/contrib/pg_stat_statements/pg_stat_statements.c
index f177eba..6d456f6 100644
--- a/contrib/pg_stat_statements/pg_stat_statements.c
+++ b/contrib/pg_stat_statements/pg_stat_statements.c
@@ -2925,6 +2925,7 @@ JumbleExpr(pgssJumbleState *jstate, Node *node)

                 /* we store the string name because RTE_CTE RTEs need it */
                 APP_JUMB_STRING(cte->ctename);
+                APP_JUMB(cte->ctematerialized);
                 JumbleQuery(jstate, castNode(Query, cte->ctequery));
             }
             break;
diff --git a/contrib/postgres_fdw/expected/postgres_fdw.out b/contrib/postgres_fdw/expected/postgres_fdw.out
index bb92d9d..8c26dd1 100644
--- a/contrib/postgres_fdw/expected/postgres_fdw.out
+++ b/contrib/postgres_fdw/expected/postgres_fdw.out
@@ -1888,7 +1888,7 @@ SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c3, t

 -- join in CTE
 EXPLAIN (VERBOSE, COSTS OFF)
-WITH t (c1_1, c1_3, c2_1) AS (SELECT t1.c1, t1.c3, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1)) SELECT c1_1, c2_1
FROMt ORDER BY c1_3, c1_1 OFFSET 100 LIMIT 10; 
+WITH t (c1_1, c1_3, c2_1) AS MATERIALIZED (SELECT t1.c1, t1.c3, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1))
SELECTc1_1, c2_1 FROM t ORDER BY c1_3, c1_1 OFFSET 100 LIMIT 10; 
                                                              QUERY PLAN
              

-------------------------------------------------------------------------------------------------------------------------------------
  Limit
@@ -1905,7 +1905,7 @@ WITH t (c1_1, c1_3, c2_1) AS (SELECT t1.c1, t1.c3, t2.c1 FROM ft1 t1 JOIN ft2 t2
                Output: t.c1_1, t.c2_1, t.c1_3
 (12 rows)

-WITH t (c1_1, c1_3, c2_1) AS (SELECT t1.c1, t1.c3, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1)) SELECT c1_1, c2_1
FROMt ORDER BY c1_3, c1_1 OFFSET 100 LIMIT 10; 
+WITH t (c1_1, c1_3, c2_1) AS MATERIALIZED (SELECT t1.c1, t1.c3, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1))
SELECTc1_1, c2_1 FROM t ORDER BY c1_3, c1_1 OFFSET 100 LIMIT 10; 
  c1_1 | c2_1
 ------+------
   101 |  101
diff --git a/contrib/postgres_fdw/sql/postgres_fdw.sql b/contrib/postgres_fdw/sql/postgres_fdw.sql
index f438165..56602a1 100644
--- a/contrib/postgres_fdw/sql/postgres_fdw.sql
+++ b/contrib/postgres_fdw/sql/postgres_fdw.sql
@@ -493,8 +493,8 @@ SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c3, t
 SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c3, t1.c1 OFFSET 100 LIMIT 10 FOR SHARE;
 -- join in CTE
 EXPLAIN (VERBOSE, COSTS OFF)
-WITH t (c1_1, c1_3, c2_1) AS (SELECT t1.c1, t1.c3, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1)) SELECT c1_1, c2_1
FROMt ORDER BY c1_3, c1_1 OFFSET 100 LIMIT 10; 
-WITH t (c1_1, c1_3, c2_1) AS (SELECT t1.c1, t1.c3, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1)) SELECT c1_1, c2_1
FROMt ORDER BY c1_3, c1_1 OFFSET 100 LIMIT 10; 
+WITH t (c1_1, c1_3, c2_1) AS MATERIALIZED (SELECT t1.c1, t1.c3, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1))
SELECTc1_1, c2_1 FROM t ORDER BY c1_3, c1_1 OFFSET 100 LIMIT 10; 
+WITH t (c1_1, c1_3, c2_1) AS MATERIALIZED (SELECT t1.c1, t1.c3, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1))
SELECTc1_1, c2_1 FROM t ORDER BY c1_3, c1_1 OFFSET 100 LIMIT 10; 
 -- ctid with whole-row reference
 EXPLAIN (VERBOSE, COSTS OFF)
 SELECT t1.ctid, t1, t2, t1.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c3, t1.c1 OFFSET 100 LIMIT 10;
diff --git a/doc/src/sgml/queries.sgml b/doc/src/sgml/queries.sgml
index 88bc189..2225255 100644
--- a/doc/src/sgml/queries.sgml
+++ b/doc/src/sgml/queries.sgml
@@ -2199,16 +2199,19 @@ SELECT n FROM t LIMIT 100;
   </para>

   <para>
-   A useful property of <literal>WITH</literal> queries is that they are evaluated
-   only once per execution of the parent query, even if they are referred to
-   more than once by the parent query or sibling <literal>WITH</literal> queries.
+   A useful property of <literal>WITH</literal> queries is that they are
+   normally evaluated only once per execution of the parent query, even if
+   they are referred to more than once by the parent query or
+   sibling <literal>WITH</literal> queries.
    Thus, expensive calculations that are needed in multiple places can be
    placed within a <literal>WITH</literal> query to avoid redundant work.  Another
    possible application is to prevent unwanted multiple evaluations of
    functions with side-effects.
-   However, the other side of this coin is that the optimizer is less able to
-   push restrictions from the parent query down into a <literal>WITH</literal> query
-   than an ordinary subquery.  The <literal>WITH</literal> query will generally be
+   However, the other side of this coin is that the optimizer is not able to
+   push restrictions from the parent query down into a multiply-referenced
+   <literal>WITH</literal> query, since that might affect all uses of the
+   <literal>WITH</literal> query's output when it should affect only one.
+   The multiply-referenced <literal>WITH</literal> query will be
    evaluated as written, without suppression of rows that the parent query
    might discard afterwards.  (But, as mentioned above, evaluation might stop
    early if the reference(s) to the query demand only a limited number of
@@ -2216,6 +2219,75 @@ SELECT n FROM t LIMIT 100;
   </para>

   <para>
+   However, if a <literal>WITH</literal> query is non-recursive and
+   side-effect-free (that is, it is a <literal>SELECT</literal> containing
+   no volatile functions) then it can be folded into the parent query,
+   allowing joint optimization of the two query levels.  By default, this
+   happens if the parent query references the <literal>WITH</literal> query
+   just once, but not if it references the <literal>WITH</literal> query
+   more than once.  You can override that decision by
+   specifying <literal>MATERIALIZED</literal> to force separate calculation
+   of the <literal>WITH</literal> query, or by specifying <literal>NOT
+   MATERIALIZED</literal> to force it to be merged into the parent query.
+   The latter choice risks duplicate computation of
+   the <literal>WITH</literal> query, but it can still give a net savings if
+   each usage of the <literal>WITH</literal> query needs only a small part
+   of the <literal>WITH</literal> query's full output.
+  </para>
+
+  <para>
+   A simple example of these rules is
+<programlisting>
+WITH w AS (
+    SELECT * FROM big_table
+)
+SELECT * FROM w WHERE key = 123;
+</programlisting>
+   This <literal>WITH</literal> query will be folded, producing the same
+   execution plan as
+<programlisting>
+SELECT * FROM big_table WHERE key = 123;
+</programlisting>
+   In particular, if there's an index on <structfield>key</structfield>,
+   it will probably be used to fetch just the rows having <literal>key =
+   123</literal>.  On the other hand, in
+<programlisting>
+WITH w AS (
+    SELECT * FROM big_table
+)
+SELECT * FROM w AS w1 JOIN w AS w2 ON w1.key = w2.ref
+WHERE w2.key = 123;
+</programlisting>
+   the <literal>WITH</literal> query will be materialized, producing a
+   temporary copy of <structname>big_table</structname> that is then
+   joined with itself — without benefit of any index.  This query
+   will be executed much more efficiently if written as
+<programlisting>
+WITH w AS NOT MATERIALIZED (
+    SELECT * FROM big_table
+)
+SELECT * FROM w AS w1 JOIN w AS w2 ON w1.key = w2.ref
+WHERE w2.key = 123;
+</programlisting>
+   so that the parent query's restrictions can be applied directly
+   to scans of <structname>big_table</structname>.
+  </para>
+
+  <para>
+   An example where <literal>NOT MATERIALIZED</literal> could be
+   undesirable is
+<programlisting>
+WITH w AS (
+    SELECT key, very_expensive_function(val) as f FROM some_table
+)
+SELECT * FROM w AS w1 JOIN w AS w2 ON w1.f = w2.f;
+</programlisting>
+   Here, materialization of the <literal>WITH</literal> query ensures
+   that <function>very_expensive_function</function> is evaluated only
+   once per table row, not twice.
+  </para>
+
+  <para>
    The examples above only show <literal>WITH</literal> being used with
    <command>SELECT</command>, but it can be attached in the same way to
    <command>INSERT</command>, <command>UPDATE</command>, or <command>DELETE</command>.
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 4db8142..7035c95 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -72,7 +72,7 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="parameter">expression</replac

 <phrase>and <replaceable class="parameter">with_query</replaceable> is:</phrase>

-    <replaceable class="parameter">with_query_name</replaceable> [ ( <replaceable
class="parameter">column_name</replaceable>[, ...] ) ] AS ( <replaceable class="parameter">select</replaceable> |
<replaceableclass="parameter">values</replaceable> | <replaceable class="parameter">insert</replaceable> | <replaceable
class="parameter">update</replaceable>| <replaceable class="parameter">delete</replaceable> ) 
+    <replaceable class="parameter">with_query_name</replaceable> [ ( <replaceable
class="parameter">column_name</replaceable>[, ...] ) ] AS [ [ NOT ] MATERIALIZED ] ( <replaceable
class="parameter">select</replaceable>| <replaceable class="parameter">values</replaceable> | <replaceable
class="parameter">insert</replaceable>| <replaceable class="parameter">update</replaceable> | <replaceable
class="parameter">delete</replaceable>) 

 TABLE [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ]
 </synopsis>
@@ -93,7 +93,8 @@ TABLE [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ]
       These effectively serve as temporary tables that can be referenced
       in the <literal>FROM</literal> list.  A <literal>WITH</literal> query
       that is referenced more than once in <literal>FROM</literal> is
-      computed only once.
+      computed only once,
+      unless specified otherwise with <literal>NOT MATERIALIZED</literal>.
       (See <xref linkend="sql-with" endterm="sql-with-title"/> below.)
      </para>
     </listitem>
@@ -273,8 +274,17 @@ TABLE [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ]
    </para>

    <para>
+    The primary query and the <literal>WITH</literal> queries are all
+    (notionally) executed at the same time.  This implies that the effects of
+    a data-modifying statement in <literal>WITH</literal> cannot be seen from
+    other parts of the query, other than by reading its <literal>RETURNING</literal>
+    output.  If two such data-modifying statements attempt to modify the same
+    row, the results are unspecified.
+   </para>
+
+   <para>
     A key property of <literal>WITH</literal> queries is that they
-    are evaluated only once per execution of the primary query,
+    are normally evaluated only once per execution of the primary query,
     even if the primary query refers to them more than once.
     In particular, data-modifying statements are guaranteed to be
     executed once and only once, regardless of whether the primary query
@@ -282,12 +292,35 @@ TABLE [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ]
    </para>

    <para>
-    The primary query and the <literal>WITH</literal> queries are all
-    (notionally) executed at the same time.  This implies that the effects of
-    a data-modifying statement in <literal>WITH</literal> cannot be seen from
-    other parts of the query, other than by reading its <literal>RETURNING</literal>
-    output.  If two such data-modifying statements attempt to modify the same
-    row, the results are unspecified.
+    However, a <literal>WITH</literal> query can be marked
+    <literal>NOT MATERIALIZED</literal> to remove this guarantee.  In that
+    case, the <literal>WITH</literal> query can be folded into the primary
+    query much as though it were a simple sub-<literal>SELECT</literal> in
+    the primary query's <literal>FROM</literal> clause.  This results in
+    duplicate computations if the primary query refers to
+    that <literal>WITH</literal> query more than once; but if each such use
+    requires only a few rows of the <literal>WITH</literal> query's total
+    output, <literal>NOT MATERIALIZED</literal> can provide a net savings by
+    allowing the queries to be optimized jointly.
+    <literal>NOT MATERIALIZED</literal> is ignored if it is attached to
+    a <literal>WITH</literal> query that is recursive or is not
+    side-effect-free (i.e., is not a plain <literal>SELECT</literal>
+    containing no volatile functions).
+   </para>
+
+   <para>
+    By default, a side-effect-free <literal>WITH</literal> query is folded
+    into the primary query if it is used exactly once in the primary
+    query's <literal>FROM</literal> clause.  This allows joint optimization
+    of the two query levels in situations where that should be semantically
+    invisible.  However, such folding can be prevented by marking the
+    <literal>WITH</literal> query as <literal>MATERIALIZED</literal>.
+    That might be useful, for example, if the <literal>WITH</literal> query
+    has side-effects that are hidden from the planner.
+    <productname>PostgreSQL</productname> versions before v12 never did
+    such folding, so another possible reason for
+    using <literal>MATERIALIZED</literal> is to duplicate the pre-v12
+    behavior.
    </para>

    <para>
@@ -2087,6 +2120,12 @@ SELECT distributors.* WHERE distributors.name = 'Westward';
    <para>
     <literal>ROWS FROM( ... )</literal> is an extension of the SQL standard.
    </para>
+
+   <para>
+    The <literal>MATERIALIZED</literal> and <literal>NOT
+    MATERIALIZED</literal> options of <literal>WITH</literal> are extensions
+    of the SQL standard.
+   </para>
   </refsect2>

  </refsect1>
diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c
index 3eb7e95..3b6e206 100644
--- a/src/backend/nodes/copyfuncs.c
+++ b/src/backend/nodes/copyfuncs.c
@@ -2536,6 +2536,7 @@ _copyCommonTableExpr(const CommonTableExpr *from)

     COPY_STRING_FIELD(ctename);
     COPY_NODE_FIELD(aliascolnames);
+    COPY_SCALAR_FIELD(ctematerialized);
     COPY_NODE_FIELD(ctequery);
     COPY_LOCATION_FIELD(location);
     COPY_SCALAR_FIELD(cterecursive);
diff --git a/src/backend/nodes/equalfuncs.c b/src/backend/nodes/equalfuncs.c
index 5c4fa7d..e7b0fae 100644
--- a/src/backend/nodes/equalfuncs.c
+++ b/src/backend/nodes/equalfuncs.c
@@ -2791,6 +2791,7 @@ _equalCommonTableExpr(const CommonTableExpr *a, const CommonTableExpr *b)
 {
     COMPARE_STRING_FIELD(ctename);
     COMPARE_NODE_FIELD(aliascolnames);
+    COMPARE_SCALAR_FIELD(ctematerialized);
     COMPARE_NODE_FIELD(ctequery);
     COMPARE_LOCATION_FIELD(location);
     COMPARE_SCALAR_FIELD(cterecursive);
diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index 0fde876..9035575 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -2980,6 +2980,7 @@ _outCommonTableExpr(StringInfo str, const CommonTableExpr *node)

     WRITE_STRING_FIELD(ctename);
     WRITE_NODE_FIELD(aliascolnames);
+    WRITE_ENUM_FIELD(ctematerialized, CTEMaterialize);
     WRITE_NODE_FIELD(ctequery);
     WRITE_LOCATION_FIELD(location);
     WRITE_BOOL_FIELD(cterecursive);
diff --git a/src/backend/nodes/readfuncs.c b/src/backend/nodes/readfuncs.c
index ec6f256..be30894 100644
--- a/src/backend/nodes/readfuncs.c
+++ b/src/backend/nodes/readfuncs.c
@@ -418,6 +418,7 @@ _readCommonTableExpr(void)

     READ_STRING_FIELD(ctename);
     READ_NODE_FIELD(aliascolnames);
+    READ_ENUM_FIELD(ctematerialized, CTEMaterialize);
     READ_NODE_FIELD(ctequery);
     READ_LOCATION_FIELD(location);
     READ_BOOL_FIELD(cterecursive);
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 4465f00..67f276a 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -645,8 +645,8 @@ subquery_planner(PlannerGlobal *glob, Query *parse,
     root->partColsUpdated = false;

     /*
-     * If there is a WITH list, process each WITH query and build an initplan
-     * SubPlan structure for it.
+     * If there is a WITH list, process each WITH query and either convert it
+     * to RTE_SUBQUERY RTE(s) or build an initplan SubPlan structure for it.
      */
     if (parse->cteList)
         SS_process_ctes(root);
diff --git a/src/backend/optimizer/plan/subselect.c b/src/backend/optimizer/plan/subselect.c
index 64272dd..dde3715 100644
--- a/src/backend/optimizer/plan/subselect.c
+++ b/src/backend/optimizer/plan/subselect.c
@@ -57,6 +57,14 @@ typedef struct finalize_primnode_context
     Bitmapset  *paramids;        /* Non-local PARAM_EXEC paramids found */
 } finalize_primnode_context;

+typedef struct inline_cte_walker_context
+{
+    const char *ctename;        /* name and relative level of target CTE */
+    int            levelsup;
+    int            refcount;        /* number of remaining references */
+    Query       *ctequery;        /* query to substitute */
+} inline_cte_walker_context;
+

 static Node *build_subplan(PlannerInfo *root, Plan *plan, PlannerInfo *subroot,
               List *plan_params,
@@ -75,6 +83,10 @@ static Node *convert_testexpr_mutator(Node *node,
 static bool subplan_is_hashable(Plan *plan);
 static bool testexpr_is_hashable(Node *testexpr);
 static bool hash_ok_operator(OpExpr *expr);
+static bool contain_dml(Node *node);
+static bool contain_dml_walker(Node *node, void *context);
+static void inline_cte(PlannerInfo *root, CommonTableExpr *cte);
+static bool inline_cte_walker(Node *node, inline_cte_walker_context *context);
 static bool simplify_EXISTS_query(PlannerInfo *root, Query *query);
 static Query *convert_EXISTS_to_ANY(PlannerInfo *root, Query *subselect,
                       Node **testexpr, List **paramIds);
@@ -804,10 +816,13 @@ hash_ok_operator(OpExpr *expr)
 /*
  * SS_process_ctes: process a query's WITH list
  *
- * We plan each interesting WITH item and convert it to an initplan.
+ * Consider each CTE in the WITH list and either ignore it (if it's an
+ * unreferenced SELECT), "inline" it to create a regular sub-SELECT-in-FROM,
+ * or convert it to an initplan.
+ *
  * A side effect is to fill in root->cte_plan_ids with a list that
  * parallels root->parse->cteList and provides the subplan ID for
- * each CTE's initplan.
+ * each CTE's initplan, or a dummy ID (-1) if we didn't make an initplan.
  */
 void
 SS_process_ctes(PlannerInfo *root)
@@ -839,6 +854,44 @@ SS_process_ctes(PlannerInfo *root)
         }

         /*
+         * Consider inlining the CTE (creating RTE_SUBQUERY RTE(s)) instead of
+         * implementing it as a separately-planned CTE.
+         *
+         * We cannot inline if any of these conditions hold:
+         *
+         * 1. The user said not to (the CTEMaterializeAlways option).
+         *
+         * 2. The CTE is recursive.
+         *
+         * 3. The CTE has side-effects; this includes either not being a plain
+         * SELECT, or containing volatile functions.  Inlining might change
+         * the side-effects, which would be bad.
+         *
+         * Otherwise, we have an option whether to inline or not.  That should
+         * always be a win if there's just a single reference, but if the CTE
+         * is multiply-referenced then it's unclear: inlining adds duplicate
+         * computations, but the ability to absorb restrictions from the outer
+         * query level could outweigh that.  We do not have nearly enough
+         * information at this point to tell whether that's true, so we let
+         * the user express a preference.  Our default behavior is to inline
+         * only singly-referenced CTEs, but a CTE marked CTEMaterializeNever
+         * will be inlined even if multiply referenced.
+         */
+        if ((cte->ctematerialized == CTEMaterializeNever ||
+             (cte->ctematerialized == CTEMaterializeDefault &&
+              cte->cterefcount == 1)) &&
+            !cte->cterecursive &&
+            cmdType == CMD_SELECT &&
+            !contain_dml(cte->ctequery) &&
+            !contain_volatile_functions(cte->ctequery))
+        {
+            inline_cte(root, cte);
+            /* Make a dummy entry in cte_plan_ids */
+            root->cte_plan_ids = lappend_int(root->cte_plan_ids, -1);
+            continue;
+        }
+
+        /*
          * Copy the source Query node.  Probably not necessary, but let's keep
          * this similar to make_subplan.
          */
@@ -935,6 +988,127 @@ SS_process_ctes(PlannerInfo *root)
 }

 /*
+ * contain_dml: is any subquery not a plain SELECT?
+ *
+ * We reject SELECT FOR UPDATE/SHARE as well as INSERT etc.
+ */
+static bool
+contain_dml(Node *node)
+{
+    return contain_dml_walker(node, NULL);
+}
+
+static bool
+contain_dml_walker(Node *node, void *context)
+{
+    if (node == NULL)
+        return false;
+    if (IsA(node, Query))
+    {
+        Query       *query = (Query *) node;
+
+        if (query->commandType != CMD_SELECT ||
+            query->rowMarks != NIL)
+            return true;
+
+        return query_tree_walker(query, contain_dml_walker, context, 0);
+    }
+    return expression_tree_walker(node, contain_dml_walker, context);
+}
+
+/*
+ * inline_cte: convert RTE_CTE references to given CTE into RTE_SUBQUERYs
+ */
+static void
+inline_cte(PlannerInfo *root, CommonTableExpr *cte)
+{
+    struct inline_cte_walker_context context;
+
+    context.ctename = cte->ctename;
+    /* Start at levelsup = -1 because we'll immediately increment it */
+    context.levelsup = -1;
+    context.refcount = cte->cterefcount;
+    context.ctequery = castNode(Query, cte->ctequery);
+
+    (void) inline_cte_walker((Node *) root->parse, &context);
+
+    /* Assert we replaced all references */
+    Assert(context.refcount == 0);
+}
+
+static bool
+inline_cte_walker(Node *node, inline_cte_walker_context *context)
+{
+    if (node == NULL)
+        return false;
+    if (IsA(node, Query))
+    {
+        Query       *query = (Query *) node;
+
+        context->levelsup++;
+
+        /*
+         * Visit the query's RTE nodes after their contents; otherwise
+         * query_tree_walker would descend into the newly inlined CTE query,
+         * which we don't want.
+         */
+        (void) query_tree_walker(query, inline_cte_walker, context,
+                                 QTW_EXAMINE_RTES_AFTER);
+
+        context->levelsup--;
+
+        return false;
+    }
+    else if (IsA(node, RangeTblEntry))
+    {
+        RangeTblEntry *rte = (RangeTblEntry *) node;
+
+        if (rte->rtekind == RTE_CTE &&
+            strcmp(rte->ctename, context->ctename) == 0 &&
+            rte->ctelevelsup == context->levelsup)
+        {
+            /*
+             * Found a reference to replace.  Generate a copy of the CTE query
+             * with appropriate level adjustment for outer references (e.g.,
+             * to other CTEs).
+             */
+            Query       *newquery = copyObject(context->ctequery);
+
+            if (context->levelsup > 0)
+                IncrementVarSublevelsUp((Node *) newquery, context->levelsup, 1);
+
+            /*
+             * Convert the RTE_CTE RTE into a RTE_SUBQUERY.
+             *
+             * Historically, a FOR UPDATE clause has been treated as extending
+             * into views and subqueries, but not into CTEs.  We preserve this
+             * distinction by not trying to push rowmarks into the new
+             * subquery.
+             */
+            rte->rtekind = RTE_SUBQUERY;
+            rte->subquery = newquery;
+            rte->security_barrier = false;
+
+            /* Zero out CTE-specific fields */
+            rte->ctename = NULL;
+            rte->ctelevelsup = 0;
+            rte->self_reference = false;
+            rte->coltypes = NIL;
+            rte->coltypmods = NIL;
+            rte->colcollations = NIL;
+
+            /* Count the number of replacements we've done */
+            context->refcount--;
+        }
+
+        return false;
+    }
+
+    return expression_tree_walker(node, inline_cte_walker, context);
+}
+
+
+/*
  * convert_ANY_sublink_to_join: try to convert an ANY SubLink to a join
  *
  * The caller has found an ANY SubLink at the top level of one of the query's
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index c1faf41..56c31d9 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -479,7 +479,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 %type <list>    row explicit_row implicit_row type_list array_expr_list
 %type <node>    case_expr case_arg when_clause case_default
 %type <list>    when_clause_list
-%type <ival>    sub_type
+%type <ival>    sub_type opt_materialized
 %type <value>    NumericOnly
 %type <list>    NumericOnly_list
 %type <alias>    alias_clause opt_alias_clause
@@ -11322,17 +11322,24 @@ cte_list:
         | cte_list ',' common_table_expr        { $$ = lappend($1, $3); }
         ;

-common_table_expr:  name opt_name_list AS '(' PreparableStmt ')'
+common_table_expr:  name opt_name_list AS opt_materialized '(' PreparableStmt ')'
             {
                 CommonTableExpr *n = makeNode(CommonTableExpr);
                 n->ctename = $1;
                 n->aliascolnames = $2;
-                n->ctequery = $5;
+                n->ctematerialized = $4;
+                n->ctequery = $6;
                 n->location = @1;
                 $$ = (Node *) n;
             }
         ;

+opt_materialized:
+        MATERIALIZED                            { $$ = CTEMaterializeAlways; }
+        | NOT MATERIALIZED                        { $$ = CTEMaterializeNever; }
+        | /*EMPTY*/                                { $$ = CTEMaterializeDefault; }
+        ;
+
 opt_with_clause:
         with_clause                                { $$ = $1; }
         | /*EMPTY*/                                { $$ = NULL; }
@@ -16214,6 +16221,7 @@ makeRecursiveViewSelect(char *relname, List *aliases, Node *query)
     /* create common table expression */
     cte->ctename = relname;
     cte->aliascolnames = aliases;
+    cte->ctematerialized = CTEMaterializeDefault;
     cte->ctequery = query;
     cte->location = -1;

diff --git a/src/backend/utils/adt/ruleutils.c b/src/backend/utils/adt/ruleutils.c
index 302df16..1369602 100644
--- a/src/backend/utils/adt/ruleutils.c
+++ b/src/backend/utils/adt/ruleutils.c
@@ -5166,7 +5166,19 @@ get_with_clause(Query *query, deparse_context *context)
             }
             appendStringInfoChar(buf, ')');
         }
-        appendStringInfoString(buf, " AS (");
+        appendStringInfoString(buf, " AS ");
+        switch (cte->ctematerialized)
+        {
+            case CTEMaterializeDefault:
+                break;
+            case CTEMaterializeAlways:
+                appendStringInfoString(buf, "MATERIALIZED ");
+                break;
+            case CTEMaterializeNever:
+                appendStringInfoString(buf, "NOT MATERIALIZED ");
+                break;
+        }
+        appendStringInfoChar(buf, '(');
         if (PRETTY_INDENT(context))
             appendContextKeyword(context, "", 0, 0, 0);
         get_query_def((Query *) cte->ctequery, buf, context->namespaces, NULL,
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index addc2c2..ce8161d 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -1399,11 +1399,19 @@ typedef struct OnConflictClause
  *
  * We don't currently support the SEARCH or CYCLE clause.
  */
+typedef enum CTEMaterialize
+{
+    CTEMaterializeDefault,        /* no option specified */
+    CTEMaterializeAlways,        /* MATERIALIZED */
+    CTEMaterializeNever            /* NOT MATERIALIZED */
+} CTEMaterialize;
+
 typedef struct CommonTableExpr
 {
     NodeTag        type;
     char       *ctename;        /* query name (never qualified) */
     List       *aliascolnames;    /* optional list of column names */
+    CTEMaterialize ctematerialized; /* is this an optimization fence? */
     /* SelectStmt/InsertStmt/etc before parse analysis, Query afterwards: */
     Node       *ctequery;        /* the CTE's subquery */
     int            location;        /* token location, or -1 if unknown */
diff --git a/src/test/regress/expected/rowsecurity.out b/src/test/regress/expected/rowsecurity.out
index 1d12b01..2e17049 100644
--- a/src/test/regress/expected/rowsecurity.out
+++ b/src/test/regress/expected/rowsecurity.out
@@ -2179,7 +2179,7 @@ EXPLAIN (COSTS OFF) EXECUTE plancache_test;
    Filter: (((a % 2) = 0) AND f_leak(b))
 (2 rows)

-PREPARE plancache_test2 AS WITH q AS (SELECT * FROM z1 WHERE f_leak(b)) SELECT * FROM q,z2;
+PREPARE plancache_test2 AS WITH q AS MATERIALIZED (SELECT * FROM z1 WHERE f_leak(b)) SELECT * FROM q,z2;
 EXPLAIN (COSTS OFF) EXECUTE plancache_test2;
                    QUERY PLAN
 -------------------------------------------------
@@ -2192,7 +2192,7 @@ EXPLAIN (COSTS OFF) EXECUTE plancache_test2;
          ->  Seq Scan on z2
 (7 rows)

-PREPARE plancache_test3 AS WITH q AS (SELECT * FROM z2) SELECT * FROM q,z1 WHERE f_leak(z1.b);
+PREPARE plancache_test3 AS WITH q AS MATERIALIZED (SELECT * FROM z2) SELECT * FROM q,z1 WHERE f_leak(z1.b);
 EXPLAIN (COSTS OFF) EXECUTE plancache_test3;
                      QUERY PLAN
 -----------------------------------------------------
@@ -2826,7 +2826,7 @@ ALTER TABLE t1 ENABLE ROW LEVEL SECURITY;
 GRANT ALL ON t1 TO regress_rls_bob;
 INSERT INTO t1 (SELECT x, md5(x::text) FROM generate_series(0,20) x);
 SET SESSION AUTHORIZATION regress_rls_bob;
-WITH cte1 AS (SELECT * FROM t1 WHERE f_leak(b)) SELECT * FROM cte1;
+WITH cte1 AS MATERIALIZED (SELECT * FROM t1 WHERE f_leak(b)) SELECT * FROM cte1;
 NOTICE:  f_leak => cfcd208495d565ef66e7dff9f98764da
 NOTICE:  f_leak => c81e728d9d4c2f636f067f89cc14862c
 NOTICE:  f_leak => a87ff679a2f3e71d9181a67b7542122c
@@ -2853,7 +2853,8 @@ NOTICE:  f_leak => 98f13708210194c475687be6106a3b84
  20 | 98f13708210194c475687be6106a3b84
 (11 rows)

-EXPLAIN (COSTS OFF) WITH cte1 AS (SELECT * FROM t1 WHERE f_leak(b)) SELECT * FROM cte1;
+EXPLAIN (COSTS OFF)
+WITH cte1 AS MATERIALIZED (SELECT * FROM t1 WHERE f_leak(b)) SELECT * FROM cte1;
                    QUERY PLAN
 -------------------------------------------------
  CTE Scan on cte1
diff --git a/src/test/regress/expected/rowtypes.out b/src/test/regress/expected/rowtypes.out
index d6a1a33..14757b4 100644
--- a/src/test/regress/expected/rowtypes.out
+++ b/src/test/regress/expected/rowtypes.out
@@ -1037,7 +1037,7 @@ from (values (1,row(1,2)), (1,row(null,null)), (1,null),
 (6 rows)

 explain (verbose, costs off)
-with r(a,b) as
+with r(a,b) as materialized
   (values (1,row(1,2)), (1,row(null,null)), (1,null),
           (null,row(1,2)), (null,row(null,null)), (null,null) )
 select r, r is null as isnull, r is not null as isnotnull from r;
@@ -1050,7 +1050,7 @@ select r, r is null as isnull, r is not null as isnotnull from r;
            Output: "*VALUES*".column1, "*VALUES*".column2
 (5 rows)

-with r(a,b) as
+with r(a,b) as materialized
   (values (1,row(1,2)), (1,row(null,null)), (1,null),
           (null,row(1,2)), (null,row(null,null)), (null,null) )
 select r, r is null as isnull, r is not null as isnotnull from r;
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index e384cd2..bb10e27 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -3108,7 +3108,7 @@ explain (costs off) INSERT INTO hats VALUES ('h8', 'forbidden') RETURNING *;
 (5 rows)

 -- ensure upserting into a rule, with a CTE (different offsets!) works
-WITH data(hat_name, hat_color) AS (
+WITH data(hat_name, hat_color) AS MATERIALIZED (
     VALUES ('h8', 'green'),
         ('h9', 'blue'),
         ('h7', 'forbidden')
@@ -3122,7 +3122,8 @@ RETURNING *;
  h9         | blue
 (2 rows)

-EXPLAIN (costs off) WITH data(hat_name, hat_color) AS (
+EXPLAIN (costs off)
+WITH data(hat_name, hat_color) AS MATERIALIZED (
     VALUES ('h8', 'green'),
         ('h9', 'blue'),
         ('h7', 'forbidden')
diff --git a/src/test/regress/expected/subselect.out b/src/test/regress/expected/subselect.out
index 588d069..98feb41 100644
--- a/src/test/regress/expected/subselect.out
+++ b/src/test/regress/expected/subselect.out
@@ -1154,3 +1154,159 @@ fetch backward all in c1;
 (2 rows)

 commit;
+--
+-- Tests for CTE inlining behavior
+--
+-- Basic subquery that can be inlined
+explain (verbose, costs off)
+with x as (select * from (select f1 from subselect_tbl) ss)
+select * from x where f1 = 1;
+            QUERY PLAN
+----------------------------------
+ Seq Scan on public.subselect_tbl
+   Output: subselect_tbl.f1
+   Filter: (subselect_tbl.f1 = 1)
+(3 rows)
+
+-- Explicitly request materialization
+explain (verbose, costs off)
+with x as materialized (select * from (select f1 from subselect_tbl) ss)
+select * from x where f1 = 1;
+                QUERY PLAN
+------------------------------------------
+ CTE Scan on x
+   Output: x.f1
+   Filter: (x.f1 = 1)
+   CTE x
+     ->  Seq Scan on public.subselect_tbl
+           Output: subselect_tbl.f1
+(6 rows)
+
+-- Stable functions are safe to inline
+explain (verbose, costs off)
+with x as (select * from (select f1, now() from subselect_tbl) ss)
+select * from x where f1 = 1;
+            QUERY PLAN
+-----------------------------------
+ Seq Scan on public.subselect_tbl
+   Output: subselect_tbl.f1, now()
+   Filter: (subselect_tbl.f1 = 1)
+(3 rows)
+
+-- Volatile functions prevent inlining
+explain (verbose, costs off)
+with x as (select * from (select f1, random() from subselect_tbl) ss)
+select * from x where f1 = 1;
+                  QUERY PLAN
+----------------------------------------------
+ CTE Scan on x
+   Output: x.f1, x.random
+   Filter: (x.f1 = 1)
+   CTE x
+     ->  Seq Scan on public.subselect_tbl
+           Output: subselect_tbl.f1, random()
+(6 rows)
+
+-- SELECT FOR UPDATE cannot be inlined
+explain (verbose, costs off)
+with x as (select * from (select f1 from subselect_tbl for update) ss)
+select * from x where f1 = 1;
+                             QUERY PLAN
+--------------------------------------------------------------------
+ CTE Scan on x
+   Output: x.f1
+   Filter: (x.f1 = 1)
+   CTE x
+     ->  Subquery Scan on ss
+           Output: ss.f1
+           ->  LockRows
+                 Output: subselect_tbl.f1, subselect_tbl.ctid
+                 ->  Seq Scan on public.subselect_tbl
+                       Output: subselect_tbl.f1, subselect_tbl.ctid
+(10 rows)
+
+-- Multiply-referenced CTEs are inlined only when requested
+explain (verbose, costs off)
+with x as (select * from (select f1, now() as n from subselect_tbl) ss)
+select * from x, x x2 where x.n = x2.n;
+                QUERY PLAN
+-------------------------------------------
+ Merge Join
+   Output: x.f1, x.n, x2.f1, x2.n
+   Merge Cond: (x.n = x2.n)
+   CTE x
+     ->  Seq Scan on public.subselect_tbl
+           Output: subselect_tbl.f1, now()
+   ->  Sort
+         Output: x.f1, x.n
+         Sort Key: x.n
+         ->  CTE Scan on x
+               Output: x.f1, x.n
+   ->  Sort
+         Output: x2.f1, x2.n
+         Sort Key: x2.n
+         ->  CTE Scan on x x2
+               Output: x2.f1, x2.n
+(16 rows)
+
+explain (verbose, costs off)
+with x as not materialized (select * from (select f1, now() as n from subselect_tbl) ss)
+select * from x, x x2 where x.n = x2.n;
+                                 QUERY PLAN
+----------------------------------------------------------------------------
+ Result
+   Output: subselect_tbl.f1, now(), subselect_tbl_1.f1, now()
+   One-Time Filter: (now() = now())
+   ->  Nested Loop
+         Output: subselect_tbl.f1, subselect_tbl_1.f1
+         ->  Seq Scan on public.subselect_tbl
+               Output: subselect_tbl.f1, subselect_tbl.f2, subselect_tbl.f3
+         ->  Materialize
+               Output: subselect_tbl_1.f1
+               ->  Seq Scan on public.subselect_tbl subselect_tbl_1
+                     Output: subselect_tbl_1.f1
+(11 rows)
+
+-- Check handling of outer references
+explain (verbose, costs off)
+with x as (select * from int4_tbl)
+select * from (with y as (select * from x) select * from y) ss;
+         QUERY PLAN
+-----------------------------
+ Seq Scan on public.int4_tbl
+   Output: int4_tbl.f1
+(2 rows)
+
+explain (verbose, costs off)
+with x as materialized (select * from int4_tbl)
+select * from (with y as (select * from x) select * from y) ss;
+             QUERY PLAN
+-------------------------------------
+ CTE Scan on x
+   Output: x.f1
+   CTE x
+     ->  Seq Scan on public.int4_tbl
+           Output: int4_tbl.f1
+(5 rows)
+
+-- Ensure that we inline the currect CTE when there are
+-- multiple CTEs with the same name
+explain (verbose, costs off)
+with x as (select 1 as y)
+select * from (with x as (select 2 as y) select * from x) ss;
+ QUERY PLAN
+-------------
+ Result
+   Output: 2
+(2 rows)
+
+-- Row marks are not pushed into CTEs
+explain (verbose, costs off)
+with x as (select * from subselect_tbl)
+select * from x for update;
+                           QUERY PLAN
+----------------------------------------------------------------
+ Seq Scan on public.subselect_tbl
+   Output: subselect_tbl.f1, subselect_tbl.f2, subselect_tbl.f3
+(2 rows)
+
diff --git a/src/test/regress/sql/rowsecurity.sql b/src/test/regress/sql/rowsecurity.sql
index 38e9b38..52da276 100644
--- a/src/test/regress/sql/rowsecurity.sql
+++ b/src/test/regress/sql/rowsecurity.sql
@@ -840,10 +840,10 @@ EXPLAIN (COSTS OFF) SELECT * FROM z1 WHERE f_leak(b);
 PREPARE plancache_test AS SELECT * FROM z1 WHERE f_leak(b);
 EXPLAIN (COSTS OFF) EXECUTE plancache_test;

-PREPARE plancache_test2 AS WITH q AS (SELECT * FROM z1 WHERE f_leak(b)) SELECT * FROM q,z2;
+PREPARE plancache_test2 AS WITH q AS MATERIALIZED (SELECT * FROM z1 WHERE f_leak(b)) SELECT * FROM q,z2;
 EXPLAIN (COSTS OFF) EXECUTE plancache_test2;

-PREPARE plancache_test3 AS WITH q AS (SELECT * FROM z2) SELECT * FROM q,z1 WHERE f_leak(z1.b);
+PREPARE plancache_test3 AS WITH q AS MATERIALIZED (SELECT * FROM z2) SELECT * FROM q,z1 WHERE f_leak(z1.b);
 EXPLAIN (COSTS OFF) EXECUTE plancache_test3;

 SET ROLE regress_rls_group1;
@@ -1071,8 +1071,9 @@ INSERT INTO t1 (SELECT x, md5(x::text) FROM generate_series(0,20) x);

 SET SESSION AUTHORIZATION regress_rls_bob;

-WITH cte1 AS (SELECT * FROM t1 WHERE f_leak(b)) SELECT * FROM cte1;
-EXPLAIN (COSTS OFF) WITH cte1 AS (SELECT * FROM t1 WHERE f_leak(b)) SELECT * FROM cte1;
+WITH cte1 AS MATERIALIZED (SELECT * FROM t1 WHERE f_leak(b)) SELECT * FROM cte1;
+EXPLAIN (COSTS OFF)
+WITH cte1 AS MATERIALIZED (SELECT * FROM t1 WHERE f_leak(b)) SELECT * FROM cte1;

 WITH cte1 AS (UPDATE t1 SET a = a + 1 RETURNING *) SELECT * FROM cte1; --fail
 WITH cte1 AS (UPDATE t1 SET a = a RETURNING *) SELECT * FROM cte1; --ok
diff --git a/src/test/regress/sql/rowtypes.sql b/src/test/regress/sql/rowtypes.sql
index e6d3898..b3b0774 100644
--- a/src/test/regress/sql/rowtypes.sql
+++ b/src/test/regress/sql/rowtypes.sql
@@ -428,12 +428,12 @@ from (values (1,row(1,2)), (1,row(null,null)), (1,null),
              (null,row(1,2)), (null,row(null,null)), (null,null) ) r(a,b);

 explain (verbose, costs off)
-with r(a,b) as
+with r(a,b) as materialized
   (values (1,row(1,2)), (1,row(null,null)), (1,null),
           (null,row(1,2)), (null,row(null,null)), (null,null) )
 select r, r is null as isnull, r is not null as isnotnull from r;

-with r(a,b) as
+with r(a,b) as materialized
   (values (1,row(1,2)), (1,row(null,null)), (1,null),
           (null,row(1,2)), (null,row(null,null)), (null,null) )
 select r, r is null as isnull, r is not null as isnotnull from r;
diff --git a/src/test/regress/sql/rules.sql b/src/test/regress/sql/rules.sql
index f4ee30e..6666951 100644
--- a/src/test/regress/sql/rules.sql
+++ b/src/test/regress/sql/rules.sql
@@ -1132,7 +1132,7 @@ SELECT tablename, rulename, definition FROM pg_rules
 explain (costs off) INSERT INTO hats VALUES ('h8', 'forbidden') RETURNING *;

 -- ensure upserting into a rule, with a CTE (different offsets!) works
-WITH data(hat_name, hat_color) AS (
+WITH data(hat_name, hat_color) AS MATERIALIZED (
     VALUES ('h8', 'green'),
         ('h9', 'blue'),
         ('h7', 'forbidden')
@@ -1140,7 +1140,8 @@ WITH data(hat_name, hat_color) AS (
 INSERT INTO hats
     SELECT * FROM data
 RETURNING *;
-EXPLAIN (costs off) WITH data(hat_name, hat_color) AS (
+EXPLAIN (costs off)
+WITH data(hat_name, hat_color) AS MATERIALIZED (
     VALUES ('h8', 'green'),
         ('h9', 'blue'),
         ('h7', 'forbidden')
diff --git a/src/test/regress/sql/subselect.sql b/src/test/regress/sql/subselect.sql
index 843f511..f4093e1 100644
--- a/src/test/regress/sql/subselect.sql
+++ b/src/test/regress/sql/subselect.sql
@@ -625,3 +625,61 @@ move forward all in c1;
 fetch backward all in c1;

 commit;
+
+--
+-- Tests for CTE inlining behavior
+--
+
+-- Basic subquery that can be inlined
+explain (verbose, costs off)
+with x as (select * from (select f1 from subselect_tbl) ss)
+select * from x where f1 = 1;
+
+-- Explicitly request materialization
+explain (verbose, costs off)
+with x as materialized (select * from (select f1 from subselect_tbl) ss)
+select * from x where f1 = 1;
+
+-- Stable functions are safe to inline
+explain (verbose, costs off)
+with x as (select * from (select f1, now() from subselect_tbl) ss)
+select * from x where f1 = 1;
+
+-- Volatile functions prevent inlining
+explain (verbose, costs off)
+with x as (select * from (select f1, random() from subselect_tbl) ss)
+select * from x where f1 = 1;
+
+-- SELECT FOR UPDATE cannot be inlined
+explain (verbose, costs off)
+with x as (select * from (select f1 from subselect_tbl for update) ss)
+select * from x where f1 = 1;
+
+-- Multiply-referenced CTEs are inlined only when requested
+explain (verbose, costs off)
+with x as (select * from (select f1, now() as n from subselect_tbl) ss)
+select * from x, x x2 where x.n = x2.n;
+
+explain (verbose, costs off)
+with x as not materialized (select * from (select f1, now() as n from subselect_tbl) ss)
+select * from x, x x2 where x.n = x2.n;
+
+-- Check handling of outer references
+explain (verbose, costs off)
+with x as (select * from int4_tbl)
+select * from (with y as (select * from x) select * from y) ss;
+
+explain (verbose, costs off)
+with x as materialized (select * from int4_tbl)
+select * from (with y as (select * from x) select * from y) ss;
+
+-- Ensure that we inline the currect CTE when there are
+-- multiple CTEs with the same name
+explain (verbose, costs off)
+with x as (select 1 as y)
+select * from (with x as (select 2 as y) select * from x) ss;
+
+-- Row marks are not pushed into CTEs
+explain (verbose, costs off)
+with x as (select * from subselect_tbl)
+select * from x for update;