- "Using MRR" is no longer shown with range access.
- Instead, both range and BKA accesses will show one of the following:
= "Rowid-ordered scan"
= "Key-ordered scan"
= "Key-ordered Rowid-ordered scan"
depending on whether DS-MRR implementation will do scan keys in order, rowids in order,
or both.
- The patch also introduces a way for other storage engines/MRR implementations to
pass information to EXPLAIN output about the properties of employed MRR scans.
- In join buffering code, call join_tab_execution_startup() (#1) before we call join_tab_scan->open() (#2).
This is important with SJ-Materialization because #1 fills the materialized table, while
#2 will actually try to read the first row. Attempt to read the first row before we have
populated the materialized table would cause zero rows to be returned when actually there were matches.
When this flag is 'off' the size of the used join buffer
is taken directly from the system variable 'join_buffer_size'.
When this flag is 'on' then the size of the buffer depends
on the estimated number of rows in the partial join whose
records are to be stored in the buffer.
By default this flag is set 'on'.
An assertion failure was triggered for a 6-way join query that used two
join buffers.
The failure happened because every call of JOIN_CACHE::join_matching_records
saved and restored status of all tables that were accessed before the table
join_tab. It must do it only for those of them that follow the last table
using a join buffer.
Date: Mon, 01 Nov 2010 15:15:25 -0000
3272 Roy Lyseng 2010-11-01
Bug#52068: Optimizer generates invalid semijoin materialization plan
When MaterializeScan semijoin strategy was used and there were one
or more outer dependent tables before the semijoin tables, the scan
over the materialized table was not properly reset for each row of
the prefix outer tables.
Example: suppose we have a join order:
ot1 SJ-Mat-Scan(it2 it3) ot4
Notice that this is called a MaterializeScan, even though there is an
outer table ahead of the materialized tables. Usually a MaterializeScan
has the outer tables after the materialized table, but this is
a special (but legal) case with outer dependent tables both before and
after the materialized table.
For each qualifying row from ot1, a new scan over the materialized
table must be set up. The code failed to do that, so all scans after
the first one returned zero rows from the materialized table.
One of the hash functions employed by the BNLH join algorithm
calculates the the value of hash index for key value utilizing
every byte of the key buffer. To make this calculation valid
one has to ensure that for any key value unused bytes of the
buffer are filled with with a certain filler. We choose 0 as
a filler for these bytes.
Added an optional boolean parameter with_zerofill to the function
key_copy. If the value of the parameter is TRUE all unused bytes
of the key buffer is filled with 0.
The bug happened when BKA join algorithm used an incremental buffer
and some of the fields over which access keys were constructed
- were allocated in the previous join buffers
- were non-nullable
- belonged to inner tables of outer joins.
For such fields an offset to the field value in the record is saved
in the postfix of the record, and a zero offset indicates that the value
is null. Before the key using the field value is constructed the
value is read into the corresponding field of the record buffer and
the null bit is set for the field if the offset is 0. However if
the field is non-nullable the table->null_row must be set to 1
for null values and to 0 for non-null values to ensure proper reading
of the value from the record buffer.
The patch that introduced the new enumeration type Match_flag
for the values of match flags in the records put into join buffers
missed the necessary modifications in JOIN_CACHE::set_match_flag_if_none.
This could cause wrong results for outer joins with on expressions
only over outer tables.
Miscalculation of the minimum possible buffer size could trigger
an assert in JOIN_CACHE_HASHED::put_record when if join_buffer_size
was set to the values that is less than the length of one record to
stored in the join buffer.
It happened due to the following mistakes:
- underestimation of space needed for a key in the hash table
(we have to take into account that hash table can have more
buckets than the expected number of records).
- the value of maximum total length of all records stored in
the join buffer was not saved in the field max_used_fieldlength
by the function calc_used_field_length.
When probing into the hash table of a hashed join cache is performed
the key value should not constructed in the buffer used to build keys
in the hash tables. The constant parts of these keys copied only once,
so they should not be ever overwritten. Otherwise wrong results
can be produced by queries that employ hashed join buffers.
plans or wrong results due to the fact that JOIN_CACHE functions
ignored the possibility of interleaving materialized semijoin
tables with tables whose records were stored in join buffers.
This fixes would become mostly unnecessary if the new code of
mwl 90 was merged into 5.3 right now.
Yet the fix the code of optimize_wo_join_buffering was needed
in any case.
