MDEV-38045 support implicit names and paths to query blocks in QB_NAME hints#4777
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MDEV-38045 support implicit names and paths to query blocks in QB_NAME hints#4777
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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.
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
…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
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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.
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: