MDEV-38045: Implement QB_NAME hint with path syntax for nested query blocks by Olernov · Pull Request #4700 · MariaDB/server

Olernov · 2026-02-26T11:10:55Z

Extended QB_NAME hint to support path-based addressing of query blocks
nested within views, derived tables, and CTEs, following TiDB's syntax.

New syntax:

  QB_NAME(name, query_block_path), where
    query_block_path ::= query_block_path_element
                          [ {, query_block_path_element }... ]
    query_block_path_element ::= @ qb_path_element_select_num |
                                    qb_path_element_view_sel
    qb_path_element_view_sel ::= qb_path_element_view_name
                                  [ @ qb_path_element_select_num ]

For example,
SELECT /*+ qb_name(qb_v1, v1) */* FROM v1
The name qb_v1 is assigned to the inner query block of the view v1.

SELECT /*+ qb_name(qb_v1, v1@sel_1) */* FROM v1
Means the same but specifies that v1 is present in SELECT#1 of the current
query block.

SELECT /*+ qb_name(qb_v1, v1@sel_1 .@sel_2) */* FROM v1
This means SELECT#2 of view v1, which is present in SELECT#1 of
the current query block, gets the name qb_v1.

It is possible to specify not only view names but also derived tables
and CTE's in the path.

Views and derived tables may be nested on multiple levels, for example:
SELECT /*+ qb_name(dt2_dt1_v1_1, dt1 .dt2 .v2 .@SEL_2) no_index(t1@dt2_dt1_v1_1)*/ v1.* FROM v1 JOIN (SELECT v1.* FROM v1 JOIN (SELECT * FROM v2) dt2) dt1

Limitations:

Only SELECT statements support QB names with path. DML operations
(UPDATE, DELETE, INSERT) only support explicitly defined QB names

1. `TABLE_LIST::init_derived` is called twice during derived tables processing: first time from `mysql_derived_init` and the second time from `mysql_derived_prepare`. Both times there is a check for whether an optimizer hint or switch setting allows a derived table to be merged. However, it is not necessary to restrict merging during the initialization call, it is enough to apply hints/switch setting during the preparation phase. 2. `open_normal_and_derived_tables()` runs processing of all phases of derived tables handling with a single call. But for future commits it is required to separate DT initialization from other phases. This commit implements the separation.

@dt

This patch implements support for implicit query block (QB) names in optimizer hints, allowing hints to reference query blocks and tables within derived tables, views and CTEs without requiring explicit QB_NAME hints. Examples. -- Addressing a table inside a derived table using implicit QB name select /*+ no_index(t1@dt) */ * from (select * from t1 where a > 10) as DT; -- this is an equivalent to: select /*+ no_index(t1@dt) */ * from (select /*+ qb_name(dt)*/ * from t1 where a > 10) as DT; -- Addressing a query block corresponding to the derived table select /*+ no_bnl(@dt) */ * from (select * from t1, t2 where t.1.a > t2.a) as DT; -- View create view v1 as select * from t1 where a > 10 and b > 100; -- referencing a table inside a view by implicit QB name: select /*+ index_merge(t1@v1 idx_a, idx_b) */ * from v1, t2 where v1.a = t2.a; -- equivalent to: create view v1 as select /*+ qb_name(qb_v1) */ * from t1 where a > 10 and b > 100; select /*+ index_merge(t1@qb_v1 idx_a, idx_b) */ * from v1, t2 where v1.a = t2.a; -- CTE with aless100 as (select a from t1 where b <100) select /*+ index(t1@aless100) */ * from aless100; -- equivalent to: with aless100 as (select /*+ qb_name(aless100) */ a from t1 where b <100) select /*+ index(t1@aless100) */ * from aless100; Limitations: - Only SELECT statements support implicit QB names. DML operations (UPDATE, DELETE, INSERT) only support explicit QB names

Olernov · 2026-02-26T11:15:18Z

This PR is based on #4692 that is why three commits are displayed. Actually, only the latest commit represent the feature.

…blocks Extended QB_NAME hint to support path-based addressing of query blocks nested within views, derived tables, and CTEs, following TiDB's syntax. New syntax: QB_NAME(name, query_block_path), where query_block_path ::= query_block_path_element [ {, query_block_path_element }... ] query_block_path_element ::= @ qb_path_element_select_num | qb_path_element_view_sel qb_path_element_view_sel ::= qb_path_element_view_name [ @ qb_path_element_select_num ] For example, `SELECT /*+ qb_name(qb_v1, v1) */* FROM v1` The name `qb_v1` is assigned to the inner query block of the view `v1`. `SELECT /*+ qb_name(qb_v1, v1@sel_1) */* FROM v1` Means the same but specifies that `v1` is present in SELECT#1 of the current query block. `SELECT /*+ qb_name(qb_v1, v1@sel_1 .@sel_2) */* FROM v1` This means SELECT#2 of view `v1`, which is present in SELECT#1 of the current query block, gets the name `qb_v1`. It is possible to specify not only view names but also derived tables and CTE's in the path. Views and derived tables may be nested on multiple levels, for example: `SELECT /*+ qb_name(dt2_dt1_v1_1, dt1 .dt2 .v2 .@SEL_2) no_index(t1@dt2_dt1_v1_1)*/ v1.* FROM v1 JOIN (SELECT v1.* FROM v1 JOIN (SELECT * FROM v2) dt2) dt1` Limitations: - Only SELECT statements support QB names with path. DML operations (UPDATE, DELETE, INSERT) only support explicitly defined QB names

DaveGosselin-MariaDB

Impressive test cases and documentation. Found just a couple places where we may be able to improve test coverage.

DaveGosselin-MariaDB · 2026-02-26T15:42:02Z

sql/opt_hints_parser.cc

+    // Move up to the outer SELECT
+    sl= sl_unit->outer_select();
+  }
+  return false;


None of the new tests (main.opt_hints, main.opt_hints_impl_qb_name, main.opt_hints_qb_name_path) result in is_descendant_of_unit returning false (taking this path). Yet the code path at line 837 where target_select is NULL is exercised by other cases, so I don't think there's a problem at hand. But is it possible to construct a test case for this path?

Good observation, I'll add a test case to cover this branch.

DaveGosselin-MariaDB · 2026-02-26T15:46:03Z

sql/opt_hints_parser.cc

+    Parse_context target_pc(pc->thd, target_select);
+    target_qb= get_qb_hints(&target_pc);
+    if (!target_qb)
+      return true;


This branch also isn't exercised by the tests, and the place where this is (ultimately) called from is LEX::resolve_optimizer_hints but that call site doesn't inspect the result. I'm not sure what we should do, should we call print_warn here (or in resolve_optimizer_hints)?

DaveGosselin-MariaDB · 2026-02-26T15:58:19Z

sql/opt_hints_parser.cc

+{
+  const Query_block_path &qb_path= *this;
+  if (qb_path.is_empty()) // Simple hint, e.g. QB_NAME(qb1)
+    return;


Is it possible to exercise this code path?

DaveGosselin-MariaDB · 2026-02-26T16:25:38Z

sql/opt_hints_parser.cc

+        }

-  const Lex_ident_sys qb_name_sys= Query_block_name::to_ident_sys(pc->thd);
+        uint hint_select_num= atoi(select_num_str.str + format.length);


Is it possible for hint_select_num to be zero?

DaveGosselin-MariaDB · 2026-02-26T16:26:01Z

sql/opt_hints_parser.cc

+        */
+        st_select_lex_unit *unit= target_select->master_unit();
+        uint base_select_num= unit->first_select()->select_number;
+        uint target_select_num= base_select_num + (hint_select_num - 1);


Can we guarantee that hint_select_num - 1 >= 0 ?

Olernov added 2 commits February 24, 2026 19:38

Olernov added the MariaDB Corporation label Feb 26, 2026

Olernov requested review from DaveGosselin-MariaDB and spetrunia February 26, 2026 11:11

Olernov mentioned this pull request Feb 26, 2026

Draft: MDEV-38045 Implement QB_NAME hint with path syntax for nested query blocks #4482

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Olernov force-pushed the 13.0-MDEV-38045-qb-name-path branch from 23f0615 to b98f295 Compare February 26, 2026 14:58

Olernov force-pushed the 13.0-MDEV-38045-qb-name-path branch from b98f295 to 30a82bb Compare February 26, 2026 15:22

DaveGosselin-MariaDB reviewed Feb 26, 2026

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MDEV-38045: Implement QB_NAME hint with path syntax for nested query blocks#4700

MDEV-38045: Implement QB_NAME hint with path syntax for nested query blocks#4700
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