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https://issues.apache.org/jira/browse/DERBY-6317?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13751708#comment-13751708
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Mike Matrigali commented on DERBY-6317:
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>It does choose a valid and optimal one but because the BtreeCostController is
>really not coming into play. That is returning all rows as the >estimate row
>count. So for example, for a 7M row table, the estimate row count is 7M and
>the cost is very high because of the other >calculations done in there (ie.
>cost += (estimated_row_count * row_size) * BASE_ROW_PER_BYTECOST).
>Then FromBaseTable uses the statistics selectivity in the final cost
>computation.
>So the answer is yes it gets a "correct" plan, but though a different
>computation of the cost.
good, just wanted to make sure that there was not a bug in that area also. I
believe this is working as designed, picking
a correct plan for the "average" possible value - since this compiled plan then
can be used over and over by different values.
This is what we usually recommend to users to avoid a recompile cost on every
execution, which may be why this bug has
existed for a long time.
> Optmizer can choose the wrong path when BTreeCostController.java returns an
> estimate cost and row count of 0.0
> --------------------------------------------------------------------------------------------------------------
>
> Key: DERBY-6317
> URL: https://issues.apache.org/jira/browse/DERBY-6317
> Project: Derby
> Issue Type: Bug
> Components: SQL
> Affects Versions: 10.8.2.2
> Environment: Derby 10.8.2.2 on Oracle Solaris 10
> Reporter: Brett Bergquist
>
> The optimizer can chose the wrong path when BTreeCostController.java returns
> an estimate cost and row count of 0.0.
> Assume that you have two tables that are being joined like:
> SELECT * FROM T1, T0
> WHERE T1.ID = T0.F_ID and
> T0.ID = 3;
> Also assume that T0 has two columns, ID and F_ID and F_ID is a foreign key on
> T1.ID. Assume that T1.ID is the primary key of T1 and (T0.F_ID, T0.ID) is
> the primary key on T0. Assume that there is a non-unique index on T0.ID.
> The correct query plan for this should be to query T0 using the non-unique
> index on T0.ID and then use the foreign key value in those rows to do query
> T1 using the primary key on T1.
> With some values of T0.ID in the above query this query plan is chosen and
> works. With other values of T0.ID , the query plan does an query on T0 using
> the non-unique index on T0.ID and then does a table scan on T1.
> For example, in my case the query:
> SELECT * FROM T1, T0
> WHERE T1.ID = T0.F_ID and
> T0.ID = 22112129;
> has this query plan.
> The problem appears to be in BTreeCostController.java. When this returns the
> same value for the "left_of_start" and the "left_of_stop" (which is being
> used to estimate the number of rows and cost), then the estimate cost and row
> count becomes 0.0. When this is used in the join order of T0, T1, then the
> cost of the table scan for T1 becomes 0.0 as well.
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