How to fix Error 565 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 565 – Stack Overflow During Query Compilation

Why This Error Happens

Error 565 occurs when SQL Server’s query optimizer exceeds the available thread stack space while attempting to compile an excessively complex or deeply nested query. This is typically caused by queries containing an extreme number of nested subqueries, recursive Common Table Expressions (CTEs) without proper termination, deeply chained JOINs, or auto-generated SQL from ORM tools that produce overly complex WHERE clauses or UNION statements. The optimizer’s recursive parsing algorithm runs out of stack memory before it can finish building the execution plan.

Step-by-Step Resolution Guide

Step 1: Identify and Isolate the Offending Query

  • Check the SQL Server Error Log (Management Studio > Management > SQL Server Logs) or the Windows Event Viewer under Applications and Services Logs > MSSQLSERVER for the exact query batch ID that triggered Error 565.
  • Use sys.dm_exec_query_stats combined with sys.dm_exec_sql_text() to retrieve the last executed statement text before the crash, if the session is still cached.
  • Isolate the query in SQL Server Management Studio (SSMS) and attempt to run it standalone to confirm it reproduces the error.

Step 2: Simplify the Query Structure

  • Break large nested subqueries into temporary tables or table variables (e.g., SELECT ... INTO #TempResults) to reduce compilation depth.
  • If using recursive CTEs, add an explicit MAXRECURSION hint (e.g., OPTION (MAXRECURSION 100)) and verify the anchor/recursive member logic terminates correctly.
  • For ORM-generated queries (Entity Framework, Hibernate, etc.), review the generated SQL in C:\Program Files\Microsoft SQL Server\MSSQL16.MSSQLSERVER\MSSQL\Log\ and refactor the LINQ/HQL query to reduce chained .Where() or .Include() calls that compound into massive WHERE clauses.

Step 3: Increase Thread Stack Size (If Simplification Isn’t Feasible)

  • Open SQL Server Configuration Manager on Windows Server 2025/Windows 11.
  • Navigate to SQL Server Services > SQL Server (MSSQLSERVER) > Properties > Startup Parameters.
  • Add the trace flag -T3625 cautiously only in test environments to gather more diagnostic stack info (not a permanent production fix).
  • As a workaround, adjust the max worker threads setting via sp_configure 'max worker threads', 0; followed by RECONFIGURE; to let SQL Server auto-manage thread allocation more efficiently.

Step 4: Update SQL Server 2025 Cumulative Updates

  • Check Help > About in SSMS or run SELECT @@VERSION; to confirm your build number.
  • Download and apply the latest Cumulative Update (CU) from the Microsoft Update Catalog, as stack overflow compilation bugs in complex query plans are frequently patched in CUs.
  • Restart the SQL Server (MSSQLSERVER) service via services.msc after installation.

Alternative or Verification Steps

  • Re-run the simplified query in SSMS and confirm the execution plan generates successfully (Ctrl+M to include actual execution plan) without throwing Error 565.
  • If the error persists, use OPTION (RECOMPILE, FORCE ORDER) as a temporary hint to bypass complex optimizer decision trees, or split the single query into multiple sequential batches using temp tables to permanently avoid deep recursive compilation.