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MDEV-23908: Implement SELECT ... OFFSET ... FETCH ...

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This commit implements the standard SQL extension
OFFSET start { ROW | ROWS }
[FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } { ONLY | WITH TIES }]

To achieve this a reserved keyword OFFSET is introduced.

The general logic for WITH TIES implies:
1. The number of rows a query returns is no longer known during optimize
   phase. Adjust optimizations to no longer consider this.
2. During end_send make use of an "order Cached_item"to compare if the
   ORDER BY columns changed. Keep returning rows until there is a
   change. This happens only after we reached the row limit.
3. Within end_send_group, the order by clause was eliminated. It is
   still possible to keep the optimization of using end_send_group for
   producing the final result set.
This commit is contained in:
Vicențiu Ciorbaru
2021-03-28 21:41:50 +03:00
parent 2d595319bf
commit 299b935320
19 changed files with 2621 additions and 81 deletions
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16
sql/sql_select.h
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@ -1177,6 +1177,16 @@ public:
uint top_join_tab_count;
uint aggr_tables; ///< Number of post-join tmp tables
uint send_group_parts;
/*
This represents the number of items in ORDER BY *after* removing
all const items. This is computed before other optimizations take place,
such as removal of ORDER BY when it is a prefix of GROUP BY, for example:
GROUP BY a, b ORDER BY a
This is used when deciding to send rows, by examining the correct number
of items in the group_fields list when ORDER BY was previously eliminated.
*/
uint with_ties_order_count;
/*
True if the query has GROUP BY.
(that is, if group_by != NULL. when DISTINCT is converted into GROUP BY, it
@ -1306,6 +1316,10 @@ public:
*/
double join_record_count;
List<Item> *fields;
/* Used only for FETCH ... WITH TIES to identify peers. */
List<Cached_item> order_fields;
/* Used during GROUP BY operations to identify when a group has changed. */
List<Cached_item> group_fields, group_fields_cache;
THD *thd;
Item_sum **sum_funcs, ***sum_funcs_end;
@ -1609,6 +1623,8 @@ public:
sjm_lookup_tables= 0;
sjm_scan_tables= 0;
is_orig_degenerated= false;
with_ties_order_count= 0;
}
/* True if the plan guarantees that it will be returned zero or one row */