Hi Gerrit,
Some responses inline...
On 1/26/18 12:50 AM, [email protected] wrote:
Hello Rick,
me again. As we had a blackout of our Internet connection I had some
unexpected time. ;-)
So I tried the analysis of Derby:
CALL SYSCS_UTIL.SYSCS_SET_RUNTIMESTATISTICS(1)
CALL SYSCS_UTIL.SYSCS_SET_STATISTICS_TIMING(1)
SELECT * FROM history ORDER BY timestamp DESC FETCH FIRST 10 ROWS ONLY
VALUES SYSCS_UTIL.SYSCS_GET_RUNTIMESTATISTICS()
CALL SYSCS_UTIL.SYSCS_SET_RUNTIMESTATISTICS(0)
CALL SYSCS_UTIL.SYSCS_SET_STATISTICS_TIMING(0)
The result of the "VALUES" command was:
> 1
> ----------------------------------------------------
> Statement Name: \n null\nStatement Text: \n SELECT
This is indeed not what you'd expect. The output has been truncated by
the default display width for the ij tool. You need to set
MAXIMUMDISPLAYWIDTH to something very large. Here's an example of how to
do this:
ij version 10.15
ij> connect 'jdbc:derby:memory:db;create=true';
ij> create table foo( a int );
0 rows inserted/updated/deleted
ij> MAXIMUMDISPLAYWIDTH 7000;
ij> CALL SYSCS_UTIL.SYSCS_SET_RUNTIMESTATISTICS(1);
0 rows inserted/updated/deleted
ij> CALL SYSCS_UTIL.SYSCS_SET_STATISTICS_TIMING(1);
0 rows inserted/updated/deleted
ij> select count(*) from foo where a <> 0;
1
-----------
0
1 row selected
ij> values SYSCS_UTIL.SYSCS_GET_RUNTIMESTATISTICS();
1
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Statement Name:
null
Statement Text:
select count(*) from foo where a <> 0
Parse Time: 25
Bind Time: 18
Optimize Time: 21
Generate Time: 15
Compile Time: 79
Execute Time: 1
Begin Compilation Timestamp : 2018-01-27 13:27:10.819
End Compilation Timestamp : 2018-01-27 13:27:10.898
Begin Execution Timestamp : 2018-01-27 13:27:10.963
End Execution Timestamp : 2018-01-27 13:27:11.003
Statement Execution Plan Text:
Project-Restrict ResultSet (4):
Number of opens = 1
Rows seen = 1
Rows filtered = 0
restriction = false
projection = true
constructor time (milliseconds) = 0
open time (milliseconds) = 1
next time (milliseconds) = 0
close time (milliseconds) = 0
restriction time (milliseconds) = 0
projection time (milliseconds) = 0
optimizer estimated row count: 1.00
optimizer estimated cost: 100.40
Source result set:
Scalar Aggregate ResultSet:
Number of opens = 1
Rows input = 0
constructor time (milliseconds) = 55
open time (milliseconds) = 1
next time (milliseconds) = 0
close time (milliseconds) = 25
optimizer estimated row count: 1.98
optimizer estimated cost: 100.40
Index Key Optimization = false
Source result set:
Project-Restrict ResultSet (3):
Number of opens = 1
Rows seen = 0
Rows filtered = 0
restriction = false
projection = true
constructor time (milliseconds) = 0
open time (milliseconds) = 1
next time (milliseconds) = 0
close time (milliseconds) = 25
restriction time (milliseconds) = 0
projection time (milliseconds) = 0
optimizer estimated row count: 1.98
optimizer estimated cost: 100.40
Source result set:
Table Scan ResultSet for FOO at read committed isolation
level using instantaneous share row locking chosen by the optimizer
Number of opens = 1
Rows seen = 0
Rows filtered = 0
Fetch Size = 16
constructor time (milliseconds) = 0
open time (milliseconds) = 59
next time (milliseconds) = 0
close time (milliseconds) = 25
scan information:
Bit set of columns fetched=All
Number of columns fetched=1
Number of pages visited=1
Number of rows qualified=0
Number of rows visited=0
Scan type=heap
start position:
null
stop position:
null
qualifiers:
Column[0][0] Id: 0
Operator: =
Ordered nulls: false
Unknown return value: true
Negate comparison result: true
optimizer estimated row count: 1.98
optimizer estimated cost: 100.40
Not quite what I would have expected. But then I read on a different
page of the documentation that I have to declare where the information
of the analysis should be stored.
So I did ('myapp' is the schema of my app):
CALL SYSCS_UTIL.SYSCS_SET_RUNTIMESTATISTICS(1)
CALL SYSCS_UTIL.SYSCS_SET_STATISTICS_TIMING(1)
CALL SYSCS_UTIL.SYSCS_SET_XPLAIN_SCHEMA('myapp')
Executing that command I get:
> ExampleExceptionFormatter: exception message was:
'SYSCS_UTIL.SYSCS_SET_XPLAIN_SCHEMA' is not recognized as a function
or procedure.
I have a Derby 10.14 and took that command from the documentation of
that version. So I'm not sure why it gives me this error message.
Regards,
Gerrit
*Von:*Hohl, Gerrit
*Gesendet:* Freitag, 26. Januar 2018 09:11
*An:* 'Derby Discussion'
*Betreff:* AW: Derby Scheduler and FETCH FIRST question
Hello Rick,
and thanks for your reply.
I will try what you've written as soon as I have time for it.
Unfortunately I already switch to another project and I don't know
when I will get the time to have a look on this again (blame my
superiors ;-) ).
2) is surely a good idea.
1) Did you also read my 2nd mail?
I also tried using a subselect, so I have a WHERE clause. I had the
same idea as you that the scheduler might not recognize the ORDER BY
and FETCH FIRST.
It was faster, but still not what I would have expected. I've worked a
lot with Borland Interbase / Firebird, MySQL and especially with
PostgreSQL.
And PostgreSQL would have done a lot faster than this.
By the way: PostgreSQL also has a more easy to use approach in aspect
of analysis: https://www.postgresql.org/docs/9.6/static/using-explain.html
Would be great of Derby would offer something similar.
Regards,
Gerrit
*Von:*Rick Hillegas [mailto:[email protected]]
*Gesendet:* Freitag, 26. Januar 2018 00:39
*An:* [email protected] <mailto:[email protected]>
*Betreff:* Re: Derby Scheduler and FETCH FIRST question
On 1/24/18 4:45 AM, [email protected] <mailto:[email protected]> wrote:
Hello everyone,
I'm using Apache Derby v10.14.1.0 and having some problems using
the FETCH FIRST clauses.
https://db.apache.org/derby/docs/10.14/ref/rrefsqljoffsetfetch.html
I'm accessing the database using the Derby Embedded driver.
I have a table which contains some indexes as well as some fields
and a BLOB field. The table is somewhat big (means many rows, ~13 GB).
I'm using a query like this (timestamp has an index):
SELECT * FROM history ORDER BY timestamp DESC FETCH FIRST 10 ROWS ONLY
The query takes ages (about 27 minutes for that ~13 GB table) and
I can see how Derby slowly fills up my harddisk.
And a look in the "tmp" folder of the database shows several
".tmp" files.
First I get several files having 10 MB, then I get two big files
having 5 GB, then the 10 MB files are deleted, then the 5 GB files
are deleted and finally I get the result.
As I thought something is wrong with my application I also did the
same query on the same database and table using SQuirreL v3.8.1.
But the result is the same.
I would have expected that the scheduler of Derby would first look
at the timestamp column / index (which should be sorted), taking
the first 10 values from there and
finally reading the first 10 rows matching these values.
Instead it seems that it first processes the " SELECT * FROM
history" part (as memory is not sufficient it swaps it to the
harddisk), orders it and takes the first 10 elements.
Is that correct?
And if that is correct, where is the benefit of FETCH FIRST -
beside that maybe not that much data is transferred (maybe only
interesting if you use Derby not by the Embedded Driver because of
the TCP/IP connection)?
Regards,
Gerrit
Hi Gerrit,
Can you share table and index DDL for this problem as well as the
query plan which Derby chose for the query? See the section on
"Working with RunTimeStatistics" in the Derby Tuning Guide:
http://db.apache.org/derby/docs/10.14/tuning/index.html
It may be that Derby did not choose the index. That in turn, may have
happened for 2 reasons:
1) You're selecting all of the columns in the table and there is no
filtering WHERE clause. That reduces the likelihood that Derby will
pick an indexed access path since the optimizer sees this as a full
table scan.
2) I don't think that any optimizer support was built for the FETCH
FIRST clause. That's worth filing a performance bug for. I think that
the FETCH FIRST clause is only applied at execution time in order to
short-circuit the number of rows which are returned.
Thanks,
-Rick
