kadirozde commented on pull request #1256:
URL: https://github.com/apache/phoenix/pull/1256#issuecomment-1023603041
> @kadirozde @lhofhansl FYI.
>
> 1.You said "Phoenix client does not use a global index for the queries
with the columns that are not covered by the global index" is not right , In
QueryOptimizer.addPlan, for the sql with the columns that are not covered by
the global index, if user specify a Index Hint and there exists where clause,
the sql would be rewritten as "SELECT /*+ NO_INDEX _/ K,V1,V2 FROM T WHERE ("K"
IN ((SELECT /_+ INDEX(T IDX) */ ":K" FROM "IDX" WHERE "0:V1" = 'bar')) AND V2 =
'foo') " (k is pk of T , v1 is in IDX and v2 is not), you may consider
compatibility with exising code.
>
> 2.Whether or not scaning the gobal index and retrieving the corresponding
rows from the data table is better than just scaning the data table is a
complex problem, because there are many factors we need to consider such as
Network cost, random disk access cost , data distribution , column selective
etc. You said "It is expected that such performance improvement will happen
when the index row key prefix length is greater than the data row key prefix
length for a given query" is extremely insufficient. Lack of a CBO framework in
Phoenix, seems that it is sensible to be conservative, I think it is better to
left whether or not select this strategy to user by user specifying the Index
Hint just as the existing code.
@comnetwork @lhofhansl, I have updated the PR such that the uncovered global
indexes will be used only when the index hint is provided as @comnetwork
suggested.
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