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https://issues.apache.org/jira/browse/PHOENIX-3023?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15348122#comment-15348122
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James Taylor commented on PHOENIX-3023:
---------------------------------------
Looks good, [~samarthjain]. Couple of questions and some minor nits:
- If there's a WHERE clause (or even if the stats are out of date), you might
need more than one scan. Is that case handled?
- Remove this (and any others) or replace with logging:
{code}
+ System.out.println("Found last scan");
{code}
- Maybe have ScanLocation.isFirstScan() method for better readability here:
{code}
+ if ((isReverse && scanLocation.isLastScan()) ||
(!isReverse && scanLocation.getInnerListIndex() == 0)) {
{code}
- Any consideration for if we're doing round robin and we don't call peek here?
> Slow performance when limit queries are executed in parallel by default
> -----------------------------------------------------------------------
>
> Key: PHOENIX-3023
> URL: https://issues.apache.org/jira/browse/PHOENIX-3023
> Project: Phoenix
> Issue Type: Bug
> Affects Versions: 4.8.0
> Reporter: Mujtaba Chohan
> Assignee: Samarth Jain
> Attachments: PHOENIX-3023.patch
>
>
> After
> [this|https://git-wip-us.apache.org/repos/asf?p=phoenix.git;a=commit;h=54362430d71be788d515944573572624628a09b6]
> commit, limit queries are executed in parallel which causes performance to
> be ~5-10x slower. Providing a serial hint fixes it though.
> After commit:
> {code}
> select * from WIDE_PK order by mypk DESC limit 1; // this takes ~400ms
> CLIENT 1280-CHUNK 1996304 ROWS 6380181208 BYTES PARALLEL 4-WAY REVERSE FULL
> SCAN OVER WIDE_PK SERVER 1 ROW LIMIT CLIENT MERGE SORT CLIENT 1 ROW LIMIT
> {code}
> Before commit:
> {code}
> select * from WIDE_PK order by mypk DESC limit 1; // this takes ~40ms
> CLIENT 1280-CHUNK 1996304 ROWS 6380181208 BYTES SERIAL 4-WAY REVERSE FULL
> SCAN OVER WIDE_PK SERVER 1 ROW LIMIT CLIENT MERGE SORT CLIENT 1 ROW LIMIT
> {code}
> Test was done on a single node machine running HBase 0.98.17. DDL used was
> {code}CREATE TABLE WIDE_PK (MYPK CHAR(500) NOT NULL PRIMARY KEY,CF.column1
> INTEGER,CF.column2 INTEGER,CF.column3 INTEGER,CF.column4 INTEGER,CF.column5
> INTEGER) SALT_BUCKETS=4 with phoenix.stats.guidepost.width of 5000000
> {code}
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