[
https://issues.apache.org/jira/browse/SPARK-59354?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
ASF GitHub Bot updated SPARK-59354:
-----------------------------------
Labels: pull-request-available (was: )
> Derive a length guard from LIKE patterns with '_' wildcards
> -----------------------------------------------------------
>
> Key: SPARK-59354
> URL: https://issues.apache.org/jira/browse/SPARK-59354
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 4.1.0
> Reporter: David Mollitor
> Priority: Minor
> Labels: pull-request-available
>
> h3. What
> A {{LIKE}} pattern containing the {{_}} wildcard (which matches exactly one
> code point) is not
> simplified today – {{LikeSimplification}} leaves it as a full per-row regex.
> Since {{_}}
> constrains length, this derives a code-point {*}length guard{*}:
> * no {{%}} -> exact length: {{col LIKE 'a_c'}} implies {{{}Length(col) =
> 3{}}};
> * with {{%}} -> lower bound: {{col LIKE 'a_b%'}} implies {{{}Length(col) >=
> 3{}}};
> where N is the number of non-{{{}%{}}} code points in the pattern.
> When the pattern has no literals (only {{{}_{}}}/{{{}%{}}}) the guard is
> exactly equivalent, so it
> *replaces* the {{{}LIKE{}}}:
> {code:java}
> col LIKE '___' ==> Length(col) = 3
> col LIKE '_%' ==> Length(col) >= 1
> {code}
> When the pattern also has literals, the guard is only a necessary condition,
> so the exact
> {{LIKE}} is kept as the residual:
> {code:java}
> col LIKE 'a_c' ==> Length(col) = 3 && (col LIKE 'a_c')
> col LIKE 'a_b%' ==> Length(col) >= 3 && (col LIKE 'a_b%')
> {code}
> h3. Why are the changes needed?
> {{Length(col)}} is an O(n) check that fails fast before the regex, so short
> strings are
> rejected (and, for the literal-free cases, the regex is eliminated entirely).
> This is a
> CPU/short-circuit improvement.
> h3. Correctness
> * {{Length}} is a *code-point* count – the right measure for {{{}_{}}},
> which matches one code
> point regardless of its UTF-8 byte width (so a byte length would be wrong
> here).
> * The rewrite is valid in every context (not just predicates) and needs no
> collation gate:
> {{Length(col) = N}} agrees with {{col LIKE '...'}} even on {{null}} (both are
> null-intolerant),
> and for the literal case {{And(guard, LIKE)}} is just the {{LIKE}} conjoined
> with one of its necessary conditions, so it is equivalent to the original
> {{LIKE}} in all cases.
> * Assumes each pattern token consumes exactly one input code point, which
> holds for Spark's {{LIKE}} (Java-regex simple, 1:1 case folding). Idempotency
> under the fixed-point optimizer batch is maintained via a {{TreeNodeTag}} on
> the residual {{{}Like{}}}.
--
This message was sent by Atlassian Jira
(v8.20.10#820010)
---------------------------------------------------------------------
To unsubscribe, e-mail: [email protected]
For additional commands, e-mail: [email protected]
