On 15/02/2021 21:19, Rick Hillegas wrote:
It looks like the subquery isn't doing any good. I would have hoped that
the optimizer would be smart enough to join the subquery result to the
primary key on id and then use the resulting row positions to probe into
the heap. But the plan is just joining the subquery result to a full
table scan. I don't understand why the subquery plan is so convoluted.
However, it looks like you eliminated the memory-exhausting sort.
Here for comparison is the result of the original query, which takes
about the same time despite requiring a sort. I still don't understand
why the descending index on time is being ignored:
From "Tuning Derby"
(https://db.apache.org/derby/docs/10.8/tuning/ctunoptimz56859.html):
"Usually, sorting requires an extra step to put the data into the right
order. This extra step can be avoided for data that are already in the
right order. *For example, if a single-table query has an ORDER BY on a
single column, and there is an index on that column*, sorting can be
avoided if Derby uses the index as the access path."
The query below seems to me to satisfy that requirement.
Statement Name:
null
Statement Text:
SELECT DateTimeFormat(t_time,null) AS t_time,
facility,event,details,name,username,sector,item
FROM system_log_view
ORDER BY time DESC
NULLS LAST
FETCH NEXT 20 ROWS ONLY
Parse Time: 0
Bind Time: 0
Optimize Time: 0
Generate Time: 0
Compile Time: 0
Execute Time: 0
Begin Compilation Timestamp : null
End Compilation Timestamp : null
Begin Execution Timestamp : null
End Execution Timestamp : null
Statement Execution Plan Text:
Scroll Insensitive ResultSet:
Number of opens = 1
Rows seen = 20
Number of reads from hash table = 20
Number of writes to hash table = 20
constructor time (milliseconds) = 0
open time (milliseconds) = 0
next time (milliseconds) = 0
close time (milliseconds) = 0
optimizer estimated row count: 261301.00
optimizer estimated cost: 269004.12
Source result set:
Row Count (1):
Number of opens = 1
Rows seen = 20
Rows filtered = 0
constructor time (milliseconds) = 0
open time (milliseconds) = 0
next time (milliseconds) = 0
close time (milliseconds) = 0
optimizer estimated row count: 261301.00
optimizer estimated cost: 269004.12
Source result set:
Project-Restrict ResultSet (5):
Number of opens = 1
Rows seen = 20
Rows filtered = 0
restriction = false
projection = true
constructor time (milliseconds) = 0
open time (milliseconds) = 0
next time (milliseconds) = 0
close time (milliseconds) = 0
restriction time (milliseconds) = 0
projection time (milliseconds) = 0
optimizer estimated row count: 261301.00
optimizer estimated cost: 269004.12
Source result set:
Sort ResultSet:
Number of opens = 1
Rows input = 261297
Rows returned = 20
Eliminate duplicates = false
In sorted order = false
Sort information:
Number of merge runs=18
Number of rows input=261297
Number of rows output=261297
Size of merge runs=[14335, 14335, 14335, 14335, 14335,
14335, 14335, 14335, 14335, 14335, 14335, 14335, 14335, 14335, 14335,
14335, 14335, 14335]
Sort type=external
constructor time (milliseconds) = 0
open time (milliseconds) = 0
next time (milliseconds) = 0
close time (milliseconds) = 0
optimizer estimated row count: 261301.00
optimizer estimated cost: 269004.12
Source result set:
Project-Restrict ResultSet (3):
Number of opens = 1
Rows seen = 261297
Rows filtered = 0
restriction = false
projection = true
constructor time (milliseconds) = 0
open time (milliseconds) = 0
next time (milliseconds) = 0
close time (milliseconds) = 0
restriction time (milliseconds) = 0
projection time (milliseconds) = 0
optimizer estimated row count: 261301.00
optimizer estimated cost: 269004.12
Source result set:
Table Scan ResultSet for SYSTEM_LOG at read
uncommitted isolation level using share row locking chosen by the optimizer
Number of opens = 1
Rows seen = 261297
Rows filtered = 0
Fetch Size = 1
constructor time (milliseconds) = 0
open time (milliseconds) = 0
next time (milliseconds) = 0
close time (milliseconds) = 0
next time in milliseconds/row = 0
scan information:
Bit set of columns fetched={1, 2, 3, 4, 5, 6, 7, 8}
Number of columns fetched=8
Number of pages visited=1791
Number of rows qualified=261297
Number of rows visited=261483
Scan type=heap
start position:
null
stop position:
null
qualifiers:
None
optimizer estimated row count: 261301.00
optimizer estimated cost: 269004.12
--
John English
