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.
