This has been back-ported from 6.0 as the problems proved to afflict
5.1 as well.
The fix exposed two new bugs. They were reported as follows.
Bug no 52174: Sometimes wrong plan when reading a MAX value
from non-NULL index
Bug no 52173: Reading NULL value from non-NULL index gives wrong
result in embedded server
Both bugs taken together affect a much smaller class of queries than #47762,
so the fix stays for now.
NULL column for NULL
The optimization to read MIN() and MAX() values from an
index did not properly handle comparisons with NULL
values. Fixed by giving up the particular optimization step
if there are non-NULL safe comparisons with NULL values, as
the result is NULL anyway.
Also, Oracle copyright notice was added to all files.
Queries optimized with GROUP_MIN_MAX didn't cleanup KEYREAD
optimization properly. As a result subsequent queries may
return incomplete rows (fields are initialized to default
values).
WHERE conditions
check_group_min_max() checks if the loose index scan
optimization is applicable for a given WHERE condition, that is
if the MIN/MAX attribute participates only in range predicates
comparing the corresponding field with constants.
The problem was that it considered the whole predicate suitable
for the loose index scan optimization as soon as it encountered
a constant as a predicate argument. This is obviously wrong for
cases when a constant is the first argument of a predicate
which does not satisfy the above condition.
Fixed check_group_min_max() so that all arguments of the input
predicate are considered to decide if it passes the test, even
though a constant has already been encountered.
results in server crash
check_group_min_max_predicates() assumed the input condition
item to be one of COND_ITEM, SUBSELECT_ITEM, or FUNC_ITEM.
Since a condition of the form "field" is also a valid condition
equivalent to "field <> 0", using such a condition in a query
where the loose index scan was chosen resulted in a debug
assertion failure.
Fixed by handling conditions of the FIELD_ITEM type in
check_group_min_max_predicates().
without error
When using quick access methods for searching rows in UPDATE or
DELETE there was no check if a fatal error was not already sent
to the client while evaluating the quick condition.
As a result a false OK (following the error) was sent to the
client and the error was thus transformed into a warning.
Fixed by checking for errors sent to the client during
SQL_SELECT::check_quick() and treating them as real errors.
Fixed a wrong test case in group_min_max.test
Fixed a wrong return code in mysql_update() and mysql_delete()
WHERE and GROUP BY clause
Loose index scan may use range conditions on the argument of
the MIN/MAX aggregate functions to find the beginning/end of
the interval that satisfies the range conditions in a single go.
These range conditions may have open or closed minimum/maximum
values. When the comparison returns 0 (equal) the code should
check the type of the min/max values of the current interval
and accept or reject the row based on whether the limit is
open or not.
There was a wrong composite condition on checking this and it was
not working in all cases.
Fixed by simplifying the conditions and reversing the logic.
Range analysis did not request sorted output from the storage engine,
which cause partitioned handlers to process one partition at a time
while reading key prefixes in ascending order, causing values to be
missed. Fixed by always requesting sorted order during range analysis.
This fix is introduced in 6.0 by the fix for bug no 41136.
return no rows
The algorithm of determining the best key for loose index scan is doing a loop
over the available indexes and selects the one that has the best cost.
It retrieves the parameters of the current index into a set of variables.
If the cost of using the current index is lower than the best cost so far it
copies these variables into another set of variables that contain the
information for the best index so far.
After having checked all the indexes it uses these variables (outside of the
index loop) to create the table read plan object instance.
The was a single omission : the key_infix/key_infix_len variables were used
outside of the loop without being preserved in the loop for the best index
so far.
This causes these variables to get overwritten by the next index(es) checked.
Fixed by adding variables to hold the data for the current index, passing
the new variables to the function that assigns values to them and copying
the new variables into the existing ones when selecting a new current best
index.
To avoid further such problems moved the declarations of the variables used
to keep information about the current index inside the loop's compound
statement.
used causes server crash.
When the loose index scan access method is used values of aggregated functions
are precomputed by it. Aggregation of such functions shouldn't be performed
in this case and functions should be treated as normal ones.
The create_tmp_table function wasn't taking this into account and this led to
a crash if a query has MIN/MAX aggregate functions and employs temporary table
and loose index scan.
Now the JOIN::exec and the create_tmp_table functions treat MIN/MAX aggregate
functions as normal ones when the loose index scan is used.
used causes server crash.
When the loose index scan access method is used values of aggregated functions
are precomputed by it. Aggregation of such functions shouldn't be performed
in this case and functions should be treated as normal ones.
The create_tmp_table function wasn't taking this into account and this led to
a crash if a query has MIN/MAX aggregate functions and employs temporary table
and loose index scan.
Now the JOIN::exec and the create_tmp_table functions treat MIN/MAX aggregate
functions as normal ones when the loose index scan is used.
Loose index scan does the grouping so the temp table does
not need to do it, even when sorting.
Fixed by checking if the grouping is already done before
doing sorting and grouping in a temp table and do only
sorting.
error evaluating WHERE"
DELETE with a subquery in WHERE clause would sometimes ignore subquery
evaluation error and proceed with deletion.
The fix is to check for an error after evaluation of the WHERE clause
in DELETE.
Addressed review comments.
the loose scan optimization for grouping queries was applied returned
a wrong result set when the query was used with the SQL_BIG_RESULT
option.
The SQL_BIG_RESULT option forces to use sorting algorithm for grouping
queries instead of employing a suitable index. The current loose scan
optimization is applied only for one table queries when the suitable
index is covering. It does not make sense to use sort algorithm in this
case. However the create_sort_index function does not take into account
the possible choice of the loose scan to implement the DISTINCT operator
which makes sorting unnecessary. Moreover the current implementation of
the loose scan for queries with distinct assumes that sorting will
never happen. Thus in this case create_sort_index should not call
the function filesort.
statements
Currently the optimizer evaluates loose index scan only for top-level SELECT
statements
Extend loose index scan applicability by :
- Test the applicability of loose scan for each sub-select, instead of the
whole query. This change enables loose index scan for sub-queries.
- allow non-select statements with SELECT parts (like, e.g.
CREATE TABLE .. SELECT ...) to use loose index scan.
(Mostly in DBUG_PRINT() and unused arguments)
Fixed bug in query cache when used with traceing (--with-debug)
Fixed memory leak in mysqldump
Removed warnings from mysqltest scripts (replaced -- with #)
When using index for group by and range access the server isolates
a set of ranges based on the conditions over the key parts of the
index used. Then it uses only the ranges over the GROUP BY fields to
jump from one group to another. Since the GROUP BY fields may form a
prefix over the index, we may use only a prefix of the ranges produced
by the range optimizer.
Each range contains a notion on whether it includes its border values.
The problem is that when using a range prefix, the last range is open
because it assumes that there is a range on the next keypart. Thus when
we use a prefix range as it is, it excludes all border values.
The solution is when ignoring the suffix of the range conditions
(to jump over the GROUP BY prefix only) the server must change the
remaining intervals so they always contain their borders, e.g.
if the whole range was :
(1,-inf) <= (<group_by_col>,<min_max_arg_col>) < (1, 3) we must make
(1) <= (<group_by_col>) <= (1) because (a,b) < (c1,c2) means :
a < c1 OR (a = c1 AND b < c2).
- Make the range-et-al optimizer produce E(#table records after table
condition is applied),
- Make the join optimizer use this value,
- Add "filtered" column to EXPLAIN EXTENDED to show
fraction of records left after table condition is applied
- Adjust test results, add comments
The problem was that store_top_level_join_columns() incorrectly assumed
that the left/right neighbor of a nested join table reference can be only
at the same level in the join tree.
The fix checks if the current nested join table reference has no immediate
left/right neighbor, and if so chooses the left/right neighbors of the
nearest upper level, where these references are != NULL.
When converting DISTINCT to GROUP BY where the columns are from the covering
index and they are quoted twice in the SELECT list the optimizer is creating
improper processing sequence. This is because of the fact that the columns
of the covering index are not recognized as such and treated as non-index
columns.
Generally speaking duplicate columns can safely be removed from the GROUP
BY/DISTINCT list because this will not add or remove new rows in the
resulting set. Duplicates can be removed even if they are not consecutive
(as is the case for ORDER BY, where the duplicate columns can be removed
only if they are consecutive).
So we can safely transform "SELECT DISTINCT a,a FROM ... ORDER BY a" to
"SELECT a,a FROM ... GROUP BY a ORDER BY a" instead of
"SELECT a,a FROM .. GROUP BY a,a ORDER BY a". We can even transform
"SELECT DISTINCT a,b,a FROM ... ORDER BY a,b" to
"SELECT a,b,a FROM ... GROUP BY a,b ORDER BY a,b".
The fix to this bug consists of checking for duplicate columns in the SELECT
list when constructing the GROUP BY list in transforming DISTINCT to GROUP
BY and skipping the ones that are already in.
The bug was due to a missed case in the detection of whether an index
can be used for loose scan. More precisely, the range optimizer chose
to use loose index scan for queries for which the condition(s) over
an index key part could not be pushed to the index together with the
loose scan.
As a result, loose index scan was selecting the first row in the
index with a given GROUP BY prefix, and was applying the WHERE
clause after that, while it should have inspected all rows with
the given prefix, and apply the WHERE clause to all of them.
The fix detects and skips such cases.
If check_quick_select returns non-empty range then the function cost_group_min_max
cannot return 0 as an estimate of the number of retrieved records.
Yet the function erroneously returned 0 as the estimate in some situations.
The cause of the bug was the use of end_write_group instead of end_write
in the case when ORDER BY required a temporary table, which didn't take
into account the fact that loose index scan already computes the result
of MIN/MAX aggregate functions (and performs grouping).
The solution is to call end_write instead of end_write_group and to add
the MIN/MAX functions to the list of regular functions so that their
values are inserted into the temporary table.
Loose index scan using only second part of multipart index was choosen, which
results in creating wrong keys and endless loop.
get_best_group_min_max() now allows loose index scan for distinct only if used
keyparts forms a prefix of the index.
