[jira] [Updated] (SPARK-59354) Derive a length guard from LIKE patterns with '_' wildcards

["ASF GitHub Bot (Jira)" <ji...@apache.org>]

     [ 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{}}}.



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