Bergquist, Brett wrote:
I ended up forcing the join using "--DERBY-PROPERTIES joinOrder=fixed" to
ensure that the join would go the way it should.
Here is the DDL:
CREATE TABLE CONFIGURATION_SET
(
ID INTEGER NOT NULL PRIMARY KEY,
DTYPE VARCHAR(64),
OPLOCK INTEGER NOT NULL DEFAULT 0,
CONFIGURATION_NUMBER INTEGER NOT NULL,
CREATED_BY VARCHAR(64) NOT NULL,
CREATED_ON TIMESTAMP NOT NULL,
CONFIGURABLE_HARDWARE_ID INTEGER
);
CREATE TABLE CONFIGURATION_BUNDLE
(
ID INTEGER NOT NULL PRIMARY KEY,
DTYPE VARCHAR(64),
OPLOCK INTEGER NOT NULL DEFAULT 0,
BUNDLE_NAME VARCHAR(64) NOT NULL
);
CREATE TABLE CONFIGURATIONSET_CONFIGURATIONBUNDLE
(
CONFIGURATIONBUNDLE_ID INTEGER NOT NULL,
CONFIGURATIONSET_ID INTEGER NOT NULL,
CONSTRAINT CONFIGURATIONSET_CONFIGURATIONBUNDLE_PK PRIMARY KEY
(CONFIGURATIONBUNDLE_ID,CONFIGURATIONSET_ID)
);
ALTER TABLE CONFIGURATIONSET_CONFIGURATIONBUNDLE
ADD CONSTRAINT CONFIGURATIONSET_CONFIGURATIONBUNDLE_FK_1
FOREIGN KEY (CONFIGURATIONBUNDLE_ID)
REFERENCES CONFIGURATION_BUNDLE(ID);
ALTER TABLE CONFIGURATIONSET_CONFIGURATIONBUNDLE
ADD CONSTRAINT CONFIGURATIONSET_CONFIGURATIONBUNDLE_FK_2
FOREIGN KEY (CONFIGURATIONSET_ID)
REFERENCES CONFIGURATION_SET(ID);
CREATE INDEX CONFIGURATION_BUNDLE_IX_1 ON CONFIGURATION_BUNDLE(DTYPE ASC);
CREATE INDEX CONFIGURATION_BUNDLE_IX_2 ON CONFIGURATION_BUNDLE(BUNDLE_NAME
ASC);
CREATE INDEX CONFIGURATION_SET_IX_1 ON CONFIGURATION_SET(DTYPE ASC);
A configuration set is a point in time of the configuration of a network
device. It is made up of configuration bundles which are logical organization
of configuration parameters. The bundles are named (IP configuration, SNTP
configuration, etc.) and probably 50 to 60 per configuration set. A
configuration bundle is shared among configuration sets if it is not changed
from a period of time.
So to your questions:
- The bundle name is not unique and is probably spread out over a range of 60
discrete values pretty evenly. I guess having an index on this column is not
buying much now that I think about it
- Right now the query is a string that is being dynamically built. It is not a
prepared statement. This is done using the JPA
EntityManager.createNativeQuery. So yes, it is probably being optimized every
time.
Just a naive thought, however, is that the query is being constrained by a
single value of a primary key. One would think this would have higher value
than a constraint against a index column that allows duplicate values. I am
ignorant of the way the optimizer works. Also, I prototyped the query in
SquirrelSQL, saw it was what I needed, and then put it in place in the code.
It ran for many hours correctly and then I noted a major performance hit and
investigated.
I guess the best thing on these where I know better is to use the optimizer
override to fix the plan to ensure it does not spontaneously change. I was
under the assumption that the index stats daemon would keep the statistics up
to date and prevent this kind of thing. I guess I still don't fully
understand all of this. Note that the database had hundreds of thousands of
confguration set and configuration bundle entries and while testing today, I
probably added another thousand or two. So the change on the number of rows in
the tables was probably 1 to 10 percent.
I do appreciate your time to comment on this!
Brett
I don't know why it picks this plan. I think the optimizer will
evaluate all possible combinations of plans and look for the one
with lowest cost. I don't think it does anything smart about organizing
the search space as you say. It also stops when it has spent more time
looking for plans than it thinks it current plan will take. Your 3 way
join with ~3 indexes on each does not sound like a lot of work to evaluate.
It might be interesting to look at the plan for the fixed order if the
optimizer still prints its estimates. To see if there is a problem
there that makes it think that plan is way worse that it actually is.
Your change will definitely save compile time so should help overall
performance, especially if you can't use parameterized queries which
could be compiled once and executed multiple times. Note that even if
your application does not save a prepared statement and reexecute it,
derby will cache query plans so that if you execute a query with same
text again it will use the cached compiled form.
I would agree that the bundle_name index does not help for this query,
again not sure why it was picked. For this query it is going to go
to the base table for every index lookup, so I would guess this is going
to be close to just scanning the base table with no index if the 60
different values are spread across the base table.
Unless your application needs it for
some query that wants to return bundle_name's in order, or some query
that only wants bundle_name returned I would look at getting rid of it.
________________________________________
From: Mike Matrigali [[email protected]]
Sent: Friday, March 23, 2012 8:06 PM
To: [email protected]
Subject: Re: Why is a very bad query plan being chosen and why did it change
from good to bad?
Bergquist, Brett wrote:
Okay, I have in fact hit a problem that I think is Derby related that I need
some input on where to look.
While looking at syscs_diag.transaction table, I saw one of the queries that
needs to be performed. So on a whim, I ran this same query in SquirrelSQL and
took a look at the query execution plan. The query returns exactly 1 row.
There is an index on CORE_V1.CONFIGURATION_BUNDLE.BUNDLE_NAME.
CORE_V1.CONFIGURATION_SET.ID is the primary key.
CORE_V1.CONFIGURATIONSET_CONFIGURATIONBUNDLE has a primary key of
(CONFIGURATIONSET_ID, CONFIGURATIONBUNDLE_ID).
CORE_V1.CONFIGURATION_BUNDLE.ID is the primary key.
Here is the query plan:
Statement Name:
SQL_CURLH000C2
Statement Text:
SELECT CB.ID FROM PKG_9145E_V1.CONFIGURATION_SET CS JOIN
CORE_V1.CONFIGURATIONSET_CONFIGURATIONBUNDLE CSCB ON CSCB.CONFIGURATIONSET_ID =
CS.ID JOIN CORE_V1.CONFIGURATION_BUNDLE CB ON CB.ID =
CSCB.CONFIGURATIONBUNDLE_ID AND CB.BUNDLE_NAME = 'SOAM_SYSTEM_CFG' AND CS.ID =
898103
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:
Project-Restrict ResultSet (7):
Number of opens = 1
Rows seen = 1
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: 19561.00
optimizer estimated cost: 206733.70
Source result set:
Nested Loop Exists Join ResultSet:
Number of opens = 1
Rows seen from the left = 436409
Rows seen from the right = 1
Rows filtered = 0
Rows returned = 1
constructor time (milliseconds) = 0
open time (milliseconds) = 0
next time (milliseconds) = 0
close time (milliseconds) = 0
optimizer estimated row count: 19561.00
optimizer estimated cost: 206733.70
Left result set:
Nested Loop Join ResultSet:
Number of opens = 1
Rows seen from the left = 1
Rows seen from the right = 436409
Rows filtered = 0
Rows returned = 436409
constructor time (milliseconds) = 0
open time (milliseconds) = 0
next time (milliseconds) = 0
close time (milliseconds) = 0
optimizer estimated row count: 19561.00
optimizer estimated cost: 53033.14
Left result set:
Index Scan ResultSet for CONFIGURATION_SET using
constraint SQL081029110511950 at read committed isolation level using share row
locking chosen by the optimizer
Number of opens = 1
Rows seen = 1
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={0}
Number of columns fetched=1
Number of deleted rows visited=0
Number of pages visited=3
Number of rows qualified=1
Number of rows visited=1
Scan type=btree
Tree height=3
start position:
>= on first 1 column(s).
Ordered null semantics on the following
columns:
0
stop position:
> on first 1 column(s).
Ordered null semantics on the following
columns:
0
qualifiers:
None
optimizer estimated row count: 1.00
optimizer estimated cost: 4.71
Right result set:
Index Row to Base Row ResultSet for CONFIGURATION_BUNDLE:
Number of opens = 1
Rows seen = 436409
Columns accessed from heap = {2, 3}
constructor time (milliseconds) = 0
open time (milliseconds) = 0
next time (milliseconds) = 0
close time (milliseconds) = 0
optimizer estimated row count: 19561.00
optimizer estimated cost: 53028.43
Index Scan ResultSet for CONFIGURATION_BUNDLE
using index CONFIGURATION_BUNDLE_IX_2 at read committed isolation level using
instantaneous share row locking chosen by the optimizer
Number of opens = 1
Rows seen = 436409
Rows filtered = 0
Fetch Size = 16
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=All
Number of columns fetched=2
Number of deleted rows visited=193
Number of pages visited=3979
Number of rows qualified=436409
Number of rows visited=436603
Scan type=btree
Tree height=5
start position:
>= on first 1 column(s).
Ordered null semantics on the
following columns:
0
stop position:
> on first 1 column(s).
Ordered null semantics on the
following columns:
0
qualifiers:
None
optimizer estimated row count: 19561.00
optimizer estimated cost: 53028.43
Right result set:
Index Scan ResultSet for CONFIGURATIONSET_CONFIGURATIONBUNDLE
using constraint CONFIGURATIONSET_CONFIGURATIONBUNDLE_PK at read committed
isolation level using share row locking chosen by the optimizer
Number of opens = 436409
Rows seen = 1
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={0, 1}
Number of columns fetched=2
Number of deleted rows visited=0
Number of pages visited=2186202
Number of rows qualified=1
Number of rows visited=436409
Scan type=btree
Tree height=5
start position:
>= on first 2 column(s).
Ordered null semantics on the following columns:
0 1
stop position:
> on first 2 column(s).
Ordered null semantics on the following columns:
0 1
qualifiers:
None
optimizer estimated row count: 19561.00
optimizer estimated cost: 153700.56
It seems the optimizer decided to choose retrieve the
CORE_V1.CONFIGURATION_BUNDLE table, filter out the rows that matched the
constraint (436409), and then joined that with the other tables.
If it had chosen to retrieve the CORE_V1.CONFIGURATION_SET table first with the
constraint, it would have returned 1 row, joined this with the
CORE_V1.CONFIGURATIONSET_CONFIGURATIONBUNDLE table which would have produced
about 60 rows, and then joined with the CORE_V1.CONFIGURATION_BUNDLE table to
produce 60 rows which then would have been filtered by the constrain to 1 row,
this would have come back nearly instantaneously.
The funny thing is, the optimizer was choosing the optimal query plan for a
while today. And then I went to test a bit more and everything stopped and I
found this. Ouch that the query plan changed. More rows were added to all of
the tables today.
Note that I am running 10.8.2.2 and do have the IndexStats daemon enabled.
So any idea why the query plan would change to a non-optimal one? I almost
think I should drop the index on CORE_V1.CONFIGURATION_BUNDLE.BUNDLE_NAME so
that it will not attempt to use this index in the optimizer...
Brett
Quick answer is definitely query plans can change, and in 10.8 we will
update statistics dynamically. Those statistics are only used determine
how many duplicates on average there are for values in indexes. Even
without automatic statistics query plans can dynamically change as we
recompile them every so often to make sure they are not out of date, and
query cache can push old plans out.
Not enough info posted to fully understand the query plan. Need exact
ddl of tables involved and the indexes. Or at least exact ddl of all
tables/indexes mentioned exactly in the query plan. First thing I would
be looking for is what is the exact index on BUNDLE_NAME. We are using
CONFIGURATION_BUNDLE_IX_2.
Is CB.BUNDLE_NAME unique? If not, how many duplicates per value are
expected (rough guess), and is that distribution unusual in some way,
for example 90% of the rows are a single value.
I am not sure what is going on, but to make sure we are working on the
same issue, want to ask something about the query. I don't look at
query plans very often so not sure what gets printed when you print out
a query plan.
My question is if you are always executing a query with the exact
parameters in strings, or is the real query being executed have "?"
parameters which are substituted at execution time. This difference
causes very different paths through the optimizer.
With parameters inlined, it is likely the query is going to get
optimized every single time it is executed (at least every time any
single character in the query is different - like a different CS.ID).
But derby tends to do a way better job at it's estimates for
distributions when it has exact values. Note derby only knows about
distributions for a single value in a single index, it has nothing about
how many values come from a join based on those distributions. It just
uses some heuristics.
