fabric-lakehouse-views-perf-remediate

name	fabric-lakehouse-views-perf-remediate
description	Troubleshoot Microsoft Fabric materialized lake views (MLV) performance issues including slow refresh, incremental refresh failures, full refresh fallback, Spark job failures, lineage execution errors, data quality constraint violations, and optimal refresh configuration. Use when diagnosing MLV refresh duration, monitoring MLV runs in Monitor Hub, analyzing Spark logs for MLV failures, enabling change data feed (CDF), resolving delta table not found errors, or optimizing MLV query definitions for incremental refresh eligibility. Covers Spark SQL syntax, TBLPROPERTIES, PARTITIONED BY, CONSTRAINT CHECK ON MISMATCH DROP/FAIL, and custom environment configuration.
license	Complete terms in LICENSE.txt

Fabric Materialized Lake Views Performance remediate

Diagnose and resolve performance issues with materialized lake views (MLVs) in Microsoft Fabric lakehouses. This skill covers refresh optimization, Spark job diagnostics, data quality constraint tuning, and lineage execution remediate.

When to Use This Skill

• MLV refresh runs are taking longer than expected
• Incremental refresh is falling back to full refresh unexpectedly
• MLV lineage execution shows Failed or Skipped nodes
• Spark jobs for MLV refresh are failing with errors
• "Delta table not found" errors during MLV creation or refresh
• Data quality constraints causing unexpected pipeline failures
• Need to enable or verify optimal refresh configuration
• Custom Spark environment tuning for MLV workloads
• Monitoring and interpreting MLV run history

Prerequisites

• Microsoft Fabric workspace with Lakehouse items
• Schema-enabled lakehouse (recommended for MLV support)
• Fabric notebook for executing Spark SQL commands
• Workspace Admin or Contributor role for scheduling and monitoring
• Access to Monitor Hub for viewing MLV run details

Quick Diagnostics Checklist

Run through these checks in order when remediate MLV performance:

Step	Check	Action
1	Identify refresh mode	Verify optimal refresh toggle is enabled in lineage view
2	Check CDF status	Confirm delta.enableChangeDataFeed=true on ALL source tables
3	Review query patterns	Ensure only supported SQL constructs are used (see supported expressions)
4	Inspect run history	Open lineage view dropdown to see last 25 runs and their states
5	Check node failures	Click failed nodes in lineage to view error messages
6	Review Spark logs	Follow Detailed Logs link to Monitor Hub for Spark error logs
7	Validate data quality	Check if FAIL constraints are causing "delta table not found" errors
8	Assess source data	Determine if source has updates/deletes (forces full refresh)

Step-by-Step Workflows

Workflow 1: Diagnose Slow Refresh

1. Determine current refresh strategy - Navigate to Manage materialized lake views in the lakehouse ribbon. Check if Optimal refresh toggle is ON.
2. Verify change data feed - Run the diagnostic script to check CDF status on all source tables. See scripts/check-cdf-status.sql.
3. Check for unsupported expressions - Review the MLV definition for constructs that force full refresh. See Supported Expressions.
4. Inspect source data patterns - If source tables have UPDATE or DELETE operations, Fabric always performs full refresh regardless of CDF status.
5. Review partition strategy - Consider adding PARTITIONED BY to large MLVs to improve refresh parallelism.
6. Attach custom environment - Configure a custom Spark environment with optimized compute for heavy workloads. See references/custom-environment-guide.md.

Workflow 2: Resolve Failed Refresh Runs

1. Open lineage view - Navigate to Manage materialized lake views, select the failed run from the dropdown (last 25 runs available).
2. Identify failed node - Click the failed node in the lineage graph. Review the error message in the right-side panel.
3. Access Spark logs - Click the Detailed Logs link to navigate to Monitor Hub. Review Apache Spark error logs.
4. Common failure patterns - See references/common-failure-patterns.md for resolution steps.
5. Retry or recreate - For transient Spark failures, retry the run. For persistent errors, drop and recreate the MLV.

Workflow 3: Enable Optimal Refresh

1. Enable CDF on all source tables:

ALTER TABLE bronze.customers SET TBLPROPERTIES (delta.enableChangeDataFeed = true);
ALTER TABLE bronze.orders SET TBLPROPERTIES (delta.enableChangeDataFeed = true);

2. Verify CDF is enabled:

DESCRIBE DETAIL bronze.customers;
-- Check properties column for delta.enableChangeDataFeed=true

3. Create MLV with CDF property:

CREATE OR REPLACE MATERIALIZED LAKE VIEW silver.customer_orders
TBLPROPERTIES (delta.enableChangeDataFeed=true)
AS
SELECT 
    c.customerID,
    c.customerName,
    c.region,
    o.orderDate,
    o.orderAmount
FROM bronze.customers c INNER JOIN bronze.orders o
ON c.customerID = o.customerID;

4. Enable optimal refresh toggle - Navigate to Manage materialized lake views and verify the Optimal refresh toggle is ON (enabled by default).

Supported Expressions for Incremental Refresh

MLVs using only these constructs qualify for incremental refresh:

SQL Construct	Notes
SELECT	Only deterministic built-in functions. Non-deterministic and window functions force full refresh.
FROM	Standard table references
WHERE	Only deterministic built-in functions
INNER JOIN	Supported for incremental
WITH (CTE)	Common table expressions supported
UNION ALL	Supported
CONSTRAINT ... CHECK	Only deterministic built-in functions in constraint conditions

Unsupported constructs that force full refresh:

• LEFT JOIN, RIGHT JOIN, FULL OUTER JOIN
• Window functions (ROW_NUMBER, RANK, LAG, LEAD, etc.)
• Non-deterministic functions (current_timestamp(), rand(), etc.)
• Subqueries in SELECT or WHERE
• GROUP BY with HAVING
• DISTINCT
• User-defined functions (UDFs)

Key Spark SQL Reference

List All MLVs

SHOW MATERIALIZED LAKE VIEWS IN silver;

View MLV Definition

SHOW CREATE MATERIALIZED LAKE VIEW silver.customer_orders;

Force Full Refresh

REFRESH MATERIALIZED LAKE VIEW silver.customer_orders FULL;

Drop and Recreate

DROP MATERIALIZED LAKE VIEW silver.customer_orders;

CREATE OR REPLACE MATERIALIZED LAKE VIEW silver.customer_orders
TBLPROPERTIES (delta.enableChangeDataFeed=true)
AS
SELECT ...;

Known Issues and Limitations

Issue	Impact	Workaround
FAIL constraint + "delta table not found"	MLV creation or refresh fails	Recreate MLV using DROP action instead of FAIL
Schema names with ALL CAPS	MLV creation fails	Use lowercase or mixed-case schema names
Session-level Spark properties	Not applied during scheduled refresh	Set properties in custom environment instead
Delta time travel in MLV definition	Not supported	Remove VERSION AS OF or TIMESTAMP AS OF from queries
DML statements on MLVs	Not supported	MLVs are read-only; modify source tables instead
UDFs in SELECT	Not supported	Use built-in Spark SQL functions
Temporary views as MLV source	Not supported	Reference base tables or other MLVs directly
Cross-lakehouse lineage	Not supported	Keep all related MLVs within same lakehouse
Updating data quality constraints	Not supported	Drop and recreate the MLV with new constraints
LIKE/regex in constraint conditions	Not supported	Use simple comparison operators in constraints
Service principal authentication	Not supported for MLV APIs	Use Microsoft Entra user authentication
Single schedule per lineage	UI instability if exceeded	Maintain only one active schedule per lineage
South Central US region	Feature not available	Use a workspace in a different region

Run States Reference

State	Meaning	Action
Completed	All nodes executed successfully	No action needed
Failed	One or more nodes failed; child nodes skipped	Review failed node error, check Spark logs
Skipped	Previous run still in progress	Wait for current run to complete, or cancel it
In Progress	Run started, not yet terminal	Monitor progress in lineage view
Canceled	Manually canceled from Monitor Hub	Re-trigger if needed

remediate

Symptom	Likely Cause	Resolution
Refresh always full, never incremental	CDF not enabled on source tables	Run ALTER TABLE ... SET TBLPROPERTIES (delta.enableChangeDataFeed = true) on ALL sources
Refresh always full despite CDF enabled	Unsupported SQL constructs in MLV	Review definition for window functions, LEFT JOIN, non-deterministic functions
Refresh always full despite CDF and supported SQL	Source has UPDATE/DELETE operations	Incremental only supports append-only; redesign ETL to be append-only or accept full refresh
"Delta table not found" on create	FAIL constraint issue	Recreate MLV without FAIL; use DROP action instead
Node shows Skipped state	Parent node failed	Fix the parent node failure first
Schedule not running	Previous run still in progress	Cancel the stuck run from Monitor Hub, or wait
Environment not accessible	User lacks access to selected environment	Select an accessible environment from dropdown
Deleted environment error	Associated environment was removed	Choose a new environment in lineage settings
MLV names changed to lowercase	Expected behavior	MLV names are case-insensitive and stored as lowercase
Workspace names with spaces cause errors	Backtick syntax required	Use backtick notation:`My Workspace`.lakehouse.schema.view_name

References

• Common Failure Patterns - Detailed resolution for frequent MLV errors
• Custom Environment Guide - Spark environment tuning for MLV workloads
• Monitoring Deep Dive - Advanced Monitor Hub usage for MLV diagnostics
• CDF Status Check Script - T-SQL/Spark SQL script to audit CDF property
• MLV Health Check Script - Comprehensive MLV configuration audit
• MLV Diagnostic Template - Notebook template for systematic MLV remediate