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https://issues.apache.org/jira/browse/DRILL-5879?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16217782#comment-16217782
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ASF GitHub Bot commented on DRILL-5879:
---------------------------------------
Github user sachouche commented on a diff in the pull request:
https://github.com/apache/drill/pull/1001#discussion_r146705325
--- Diff:
exec/java-exec/src/main/codegen/templates/CastFunctionsSrcVarLenTargetVarLen.java
---
@@ -73,6 +73,9 @@ public void eval() {
out.start = in.start;
if (charCount <= length.value || length.value == 0 ) {
out.end = in.end;
+ if (charCount == (out.end - out.start)) {
+ out.asciiMode =
org.apache.drill.exec.expr.holders.VarCharHolder.CHAR_MODE_IS_ASCII; // we can
conclude this string is ASCII
--- End diff --
- As previously stated (when responding to Paul'd comment), the expression
framework is able to use the same VarCharHolder input variable when it is
shared amongst multiple expressions
- If the original column was of type var-binary, then the expression
framework will include a cast to var-char
- The cast logic will also compute the string length
- Using this information to deduce whether the string is pure ASCII or not
- UTF-8 encoding uses 1 byte for ASCII and 2, 3, or 4 for other character
sets
- If the encoded length and character length are equal, then this means
this is an ASCII string
> Optimize "Like" operator
> ------------------------
>
> Key: DRILL-5879
> URL: https://issues.apache.org/jira/browse/DRILL-5879
> Project: Apache Drill
> Issue Type: Improvement
> Components: Execution - Relational Operators
> Environment: *
> Reporter: salim achouche
> Assignee: salim achouche
> Priority: Minor
> Fix For: 1.12.0
>
>
> Query: select <column-list> from <table> where colA like '%a%' or colA like
> '%xyz%';
> Improvement Opportunities
> # Avoid isAscii computation (full access of the input string) since we're
> dealing with the same column twice
> # Optimize the "contains" for-loop
> Implementation Details
> 1)
> * Added a new integer variable "asciiMode" to the VarCharHolder class
> * The default value is -1 which indicates this info is not known
> * Otherwise this value will be set to either 1 or 0 based on the string being
> in ASCII mode or Unicode
> * The execution plan already shares the same VarCharHolder instance for all
> evaluations of the same column value
> * The asciiMode will be correctly set during the first LIKE evaluation and
> will be reused across other LIKE evaluations
> 2)
> * The "Contains" LIKE operation is quite expensive as the code needs to
> access the input string to perform character based comparisons
> * Created 4 versions of the same for-loop to a) make the loop simpler to
> optimize (Vectorization) and b) minimize comparisons
> Benchmarks
> * Lineitem table 100GB
> * Query: select l_returnflag, count(*) from dfs.`<source>` where l_comment
> not like '%a%' or l_comment like '%the%' group by l_returnflag
> * Before changes: 33sec
> * After changes : 27sec
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