sp_BlitzCache: speed up @SlowlySearchPlansFor by skipping the XML cast

Switch the @SlowlySearchPlansFor predicate from
CAST(qp.query_plan AS NVARCHAR(MAX)) LIKE ... to tqp.query_plan LIKE ...,
where tqp comes from sys.dm_exec_text_query_plan. That DMV returns the
plan as NVARCHAR(MAX) directly, so we skip the per-row XML-to-string
cast that dominated the search cost.

The extra CROSS APPLY is only added when @SlowlySearchPlansFor is set,
so non-search runs pay nothing. The existing qp (XML) apply stays in
place because the rest of sp_BlitzCache still needs it for .exist() /
.value() shredding and for @Noobsaibot.

Fixes BrentOzarULTD#3936.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: BrentOzar

sp_BlitzCache.sql

@@ -2129,7 +2129,10 @@ IF @SlowlySearchPlansFor IS NOT NULL
     BEGIN
     RAISERROR(N'Setting string search for @SlowlySearchPlansFor, so remember, this is gonna be slow', 0, 1) WITH NOWAIT;
     SET @SlowlySearchPlansFor = REPLACE((REPLACE((REPLACE((REPLACE(@SlowlySearchPlansFor, N'[', N'_')), N']', N'_')), N'^', N'_')), N'''', N'''''');
-    SET @body_where += N'       AND CAST(qp.query_plan AS NVARCHAR(MAX)) LIKE N''%' + @SlowlySearchPlansFor + N'%'' ' + @nl;
+    /* Search against sys.dm_exec_text_query_plan, which returns NVARCHAR(MAX) directly
+       and skips the per-row XML-to-string cast that CAST(qp.query_plan AS NVARCHAR(MAX)) pays.
+       Issue #3936. */
+    SET @body_where += N'       AND tqp.query_plan LIKE N''%' + @SlowlySearchPlansFor + N'%'' ' + @nl;
     END
 
 
@@ -2183,6 +2186,13 @@ SET @body += N') AS qs
        CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) AS st
        CROSS APPLY sys.dm_exec_query_plan(qs.plan_handle) AS qp ' + @nl ;
 
+IF @SlowlySearchPlansFor IS NOT NULL
+    BEGIN
+    /* Only pay for the text plan when the user is actually searching strings.
+       Issue #3936. */
+    SET @body += N'     CROSS APPLY sys.dm_exec_text_query_plan(qs.plan_handle, 0, -1) AS tqp ' + @nl ;
+    END
+
 IF @VersionShowsAirQuoteActualPlans = 1
     BEGIN
     SET @body += N'     CROSS APPLY sys.dm_exec_query_plan_stats(qs.plan_handle) AS deqps ' + @nl ;