opt: support recursive CTEs

A recursive CTE is of the form:
```
WITH RECURSIVE cte AS (
    <initial query>
  UNION ALL
    <recursive query>
)
```

Recursive CTE evaluation (paraphrased from postgres docs):
 1. Evaluate the initial query; emit the results and also save them in
    a "working" table.
 2. So long as the working table is not empty:
    - evaluate the recursive query, substituting the current contents of
      the working table for the recursive self-reference. Emit all
      resulting rows, and save them into the next iteration's working
      table.

This change adds optimizer and execution support for recursive CTEs.

Some notes for the various components:

 - optbuilder:  We build the recursive query using a `cteSource` that
   corresponds to the "working table". This code turned out to be
   tricky, in part because we want to allow non-recursive CTEs even if
   RECURSIVE is used (which is necessary when defining multiple CTEs,
   some of which are not recursive).

 - execution: the execution operator is somewhat similar to
   `applyJoinNode` (but simpler). The execbuilder provides a
   callback that can be used to regenerate the right-hand-side plan
   each time; this callback takes a reference to the working table (as
   a `bufferNode`).

 - execbuilder: to implement the callback mentioned above, we create
   an "inner" builder inside the callback which uses the same factory
   but is otherwise independent from the "outer" builder.

Fixes cockroachdb#21085.

Release note (sql change): WITH RECURSIVE is now supported.

docs/generated/sql/bnf/delete_stmt.bnf

@@ -1,2 +1,2 @@
 delete_stmt ::=
-	( ( 'WITH' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) ) |  ) 'DELETE' 'FROM' ( ( table_name opt_index_flags ) | ( table_name opt_index_flags ) table_alias_name | ( table_name opt_index_flags ) 'AS' table_alias_name ) ( ( 'WHERE' a_expr ) |  ) ( sort_clause |  ) ( limit_clause |  ) ( 'RETURNING' target_list | 'RETURNING' 'NOTHING' |  )
+	( ( 'WITH' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) | 'WITH' 'RECURSIVE' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) ) |  ) 'DELETE' 'FROM' ( ( table_name opt_index_flags ) | ( table_name opt_index_flags ) table_alias_name | ( table_name opt_index_flags ) 'AS' table_alias_name ) ( ( 'WHERE' a_expr ) |  ) ( sort_clause |  ) ( limit_clause |  ) ( 'RETURNING' target_list | 'RETURNING' 'NOTHING' |  )

docs/generated/sql/bnf/insert_stmt.bnf

@@ -1,3 +1,3 @@
 insert_stmt ::=
-	( ( 'WITH' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) ) |  ) 'INSERT' 'INTO' ( table_name | table_name 'AS' table_alias_name ) ( select_stmt | '(' ( ( ( column_name ) ) ( ( ',' ( column_name ) ) )* ) ')' select_stmt | 'DEFAULT' 'VALUES' ) ( 'RETURNING' ( ( target_elem ) ( ( ',' target_elem ) )* ) | 'RETURNING' 'NOTHING' |  )
-	| ( ( 'WITH' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) ) |  ) 'INSERT' 'INTO' ( table_name | table_name 'AS' table_alias_name ) ( select_stmt | '(' ( ( ( column_name ) ) ( ( ',' ( column_name ) ) )* ) ')' select_stmt | 'DEFAULT' 'VALUES' ) on_conflict ( 'RETURNING' ( ( target_elem ) ( ( ',' target_elem ) )* ) | 'RETURNING' 'NOTHING' |  )
+	( ( 'WITH' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) | 'WITH' 'RECURSIVE' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) ) |  ) 'INSERT' 'INTO' ( table_name | table_name 'AS' table_alias_name ) ( select_stmt | '(' ( ( ( column_name ) ) ( ( ',' ( column_name ) ) )* ) ')' select_stmt | 'DEFAULT' 'VALUES' ) ( 'RETURNING' ( ( target_elem ) ( ( ',' target_elem ) )* ) | 'RETURNING' 'NOTHING' |  )
+	| ( ( 'WITH' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) | 'WITH' 'RECURSIVE' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) ) |  ) 'INSERT' 'INTO' ( table_name | table_name 'AS' table_alias_name ) ( select_stmt | '(' ( ( ( column_name ) ) ( ( ',' ( column_name ) ) )* ) ')' select_stmt | 'DEFAULT' 'VALUES' ) on_conflict ( 'RETURNING' ( ( target_elem ) ( ( ',' target_elem ) )* ) | 'RETURNING' 'NOTHING' |  )

docs/generated/sql/bnf/select_stmt.bnf

@@ -1,3 +1,3 @@
 select_stmt ::=
-	( simple_select locking_clause | select_clause sort_clause locking_clause | select_clause ( sort_clause |  ) ( limit_clause offset_clause | offset_clause limit_clause | limit_clause | offset_clause ) locking_clause | ( 'WITH' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) ) select_clause locking_clause | ( 'WITH' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) ) select_clause sort_clause locking_clause | ( 'WITH' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) ) select_clause ( sort_clause |  ) ( limit_clause offset_clause | offset_clause limit_clause | limit_clause | offset_clause ) locking_clause )
+	( simple_select locking_clause | select_clause sort_clause locking_clause | select_clause ( sort_clause |  ) ( limit_clause offset_clause | offset_clause limit_clause | limit_clause | offset_clause ) locking_clause | ( 'WITH' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) | 'WITH' 'RECURSIVE' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) ) select_clause locking_clause | ( 'WITH' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) | 'WITH' 'RECURSIVE' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) ) select_clause sort_clause locking_clause | ( 'WITH' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) | 'WITH' 'RECURSIVE' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) ) select_clause ( sort_clause |  ) ( limit_clause offset_clause | offset_clause limit_clause | limit_clause | offset_clause ) locking_clause )
 

docs/generated/sql/bnf/stmt_block.bnf

@@ -1036,6 +1036,7 @@ opt_create_stats_options ::=
 
 with_clause ::=
 	'WITH' cte_list
+	| 'WITH' 'RECURSIVE' cte_list
 
 table_name_expr_with_index ::=
 	table_name opt_index_flags

docs/generated/sql/bnf/update_stmt.bnf

@@ -1,2 +1,2 @@
 update_stmt ::=
-	( ( 'WITH' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) ) |  ) 'UPDATE' ( ( table_name opt_index_flags ) | ( table_name opt_index_flags ) table_alias_name | ( table_name opt_index_flags ) 'AS' table_alias_name ) 'SET' ( ( ( ( column_name '=' a_expr ) | ( '(' ( ( ( column_name ) ) ( ( ',' ( column_name ) ) )* ) ')' '=' ( '(' select_stmt ')' | ( '(' ')' | '(' ( a_expr | a_expr ',' | a_expr ',' ( ( a_expr ) ( ( ',' a_expr ) )* ) ) ')' ) ) ) ) ) ( ( ',' ( ( column_name '=' a_expr ) | ( '(' ( ( ( column_name ) ) ( ( ',' ( column_name ) ) )* ) ')' '=' ( '(' select_stmt ')' | ( '(' ')' | '(' ( a_expr | a_expr ',' | a_expr ',' ( ( a_expr ) ( ( ',' a_expr ) )* ) ) ')' ) ) ) ) ) )* ) opt_from_list ( ( 'WHERE' a_expr ) |  ) ( sort_clause |  ) ( limit_clause |  ) ( 'RETURNING' target_list | 'RETURNING' 'NOTHING' |  )
+	( ( 'WITH' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) | 'WITH' 'RECURSIVE' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) ) |  ) 'UPDATE' ( ( table_name opt_index_flags ) | ( table_name opt_index_flags ) table_alias_name | ( table_name opt_index_flags ) 'AS' table_alias_name ) 'SET' ( ( ( ( column_name '=' a_expr ) | ( '(' ( ( ( column_name ) ) ( ( ',' ( column_name ) ) )* ) ')' '=' ( '(' select_stmt ')' | ( '(' ')' | '(' ( a_expr | a_expr ',' | a_expr ',' ( ( a_expr ) ( ( ',' a_expr ) )* ) ) ')' ) ) ) ) ) ( ( ',' ( ( column_name '=' a_expr ) | ( '(' ( ( ( column_name ) ) ( ( ',' ( column_name ) ) )* ) ')' '=' ( '(' select_stmt ')' | ( '(' ')' | '(' ( a_expr | a_expr ',' | a_expr ',' ( ( a_expr ) ( ( ',' a_expr ) )* ) ) ')' ) ) ) ) ) )* ) opt_from_list ( ( 'WHERE' a_expr ) |  ) ( sort_clause |  ) ( limit_clause |  ) ( 'RETURNING' target_list | 'RETURNING' 'NOTHING' |  )

docs/generated/sql/bnf/upsert_stmt.bnf

@@ -1,2 +1,2 @@
 upsert_stmt ::=
-	( ( 'WITH' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) ) |  ) 'UPSERT' 'INTO' ( table_name | table_name 'AS' table_alias_name ) ( select_stmt | '(' ( ( ( column_name ) ) ( ( ',' ( column_name ) ) )* ) ')' select_stmt | 'DEFAULT' 'VALUES' ) ( 'RETURNING' target_list | 'RETURNING' 'NOTHING' |  )
+	( ( 'WITH' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) | 'WITH' 'RECURSIVE' ( ( common_table_expr ) ( ( ',' common_table_expr ) )* ) ) |  ) 'UPSERT' 'INTO' ( table_name | table_name 'AS' table_alias_name ) ( select_stmt | '(' ( ( ( column_name ) ) ( ( ',' ( column_name ) ) )* ) ')' select_stmt | 'DEFAULT' 'VALUES' ) ( 'RETURNING' target_list | 'RETURNING' 'NOTHING' |  )

docs/generated/sql/bnf/with_clause.bnf

@@ -1,2 +1,3 @@
 with_clause ::=
 	'WITH' ( ( ( table_alias_name ( '(' ( ( name ) ( ( ',' name ) )* ) ')' |  ) 'AS' '(' preparable_stmt ')' ) ) ( ( ',' ( table_alias_name ( '(' ( ( name ) ( ( ',' name ) )* ) ')' |  ) 'AS' '(' preparable_stmt ')' ) ) )* ) ( insert_stmt | update_stmt | delete_stmt | upsert_stmt | select_stmt )
+	| 'WITH' 'RECURSIVE' ( ( ( table_alias_name ( '(' ( ( name ) ( ( ',' name ) )* ) ')' |  ) 'AS' '(' preparable_stmt ')' ) ) ( ( ',' ( table_alias_name ( '(' ( ( name ) ( ( ',' name ) )* ) ')' |  ) 'AS' '(' preparable_stmt ')' ) ) )* ) ( insert_stmt | update_stmt | delete_stmt | upsert_stmt | select_stmt )

pkg/sql/apply_join.go

@@ -294,7 +294,15 @@ func (a *applyJoinNode) Next(params runParams) (bool, error) {
 func (a *applyJoinNode) runRightSidePlan(params runParams, plan *planTop) error {
 	a.run.curRightRow = 0
 	a.run.rightRows.Clear(params.ctx)
-	rowResultWriter := NewRowResultWriter(a.run.rightRows)
+	return runPlanInsidePlan(params, plan, a.run.rightRows)
+}
+
+// runPlanInsidePlan is used to run a plan and gather the results in a row
+// container, as part of the execution of an "outer" plan.
+func runPlanInsidePlan(
+	params runParams, plan *planTop, rowContainer *rowcontainer.RowContainer,
+) error {
+	rowResultWriter := NewRowResultWriter(rowContainer)
 	recv := MakeDistSQLReceiver(
 		params.ctx, rowResultWriter, tree.Rows,
 		params.extendedEvalCtx.ExecCfg.RangeDescriptorCache,
@@ -339,7 +347,6 @@ func (a *applyJoinNode) runRightSidePlan(params runParams, plan *planTop) error
 		return recv.commErr
 	}
 	return rowResultWriter.err
-
 }
 
 func (a *applyJoinNode) Values() tree.Datums {

pkg/sql/buffer.go

@@ -67,11 +67,7 @@ func (n *bufferNode) Values() tree.Datums {
 
 func (n *bufferNode) Close(ctx context.Context) {
 	n.plan.Close(ctx)
-	// It's valid to be Closed without startExec having been called, in which
-	// case n.bufferedRows will be nil.
-	if n.bufferedRows != nil {
-		n.bufferedRows.Close(ctx)
-	}
+	n.bufferedRows.Close(ctx)
 }
 
 // scanBufferNode behaves like an iterator into the bufferNode it is