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    Avoiding Metadata Contention in Unity Catalog

    Metadata contention in Unity Catalog can occur in high-throughput Databricks environments, slowing down user queries and impacting performance across the platform. Our Finops strategy shifts left on performance. However, we have found scenarios where clients are still experiencing query slowdowns intermittently and even on optimized queries. As our client’s lakehouse footprint grows, we are seeing an emerging pattern where stress on Unity Catalog can have a downstream drag on performance across the workspace. In some cases, we have identified metadata contention in Unity Catalog as a contributor to unexpected reductions in response times after controlling for more targeted optimizations.

    How Metadata Contention Can Slow Down User Queries

    When data ingestion and transformation pipelines rely on structural metadata changes, they introduce several stress points across Unity Catalog’s architecture. These are not isolated to the ingestion job—they ripple across the control plane and affect all users.

    • Control Plane Saturation – Control plane saturation, often seen in distributed systems like Databricks, refers to the state when administrative functions (like schema updates, access control enforcement, and lineage tracking) overwhelm their processing capacity. Every structural table modification—especially those via CREATE OR REPLACE TABLE—adds to the metadata transaction load in Unity Catalog. This leads to:
      • Delayed responses from the catalog API
      • Increased latency in permission resolution
      • Slower query planning, even for unrelated queries
    • Metastore Lock Contention – Each table creation or replacement operation requires exclusive locks on the underlying metastore objects. When many jobs concurrently attempt these operations:
    • Query Plan Invalidation Cascade – CREATE OR REPLACE TABLE invalidates the current logical and physical plan cache for all compute clusters referencing the old version. This leads to:
      • Increased query planning time across clusters
      • Unpredictable performance for dashboards or interactive workloads
      • Reduced cache utilization across Spark executors
    • Schema Propagation Overhead – Structural changes to a table (e.g., column additions, type changes) must propagate to all services relying on schema consistency. This includes:
    • Multi-tenant Cross-Job Interference – Unity Catalog is a shared control plane. When one tenant (or set of jobs) aggressively replaces tables, the metadata operations can delay or block unrelated tenants. This leads to:
      • Slow query startup times for interactive users
      • Cluster spin-up delays due to metadata prefetch slowness
      • Support escalation from unrelated teams

    The CREATE OR REPLACE Reset

    In other blogs, I have said that predictive optimization is the reward for investing in good governance practices with Unity Catalog. One of the key enablers of predictive optimzation is a current, cached logical and physical plan. Every time a table is created, a new logical and physical plan for this and related tables is created. This means that ever time you execute CREATE OR REPLACE TABLE, you are back to step one for performance optimization. The DROP TABLE + CREATE TABLE pattern will have the same net result.

    This is not to say that CREATE OR REPLACE TABLE is inherently an anti-pattern. It only becomes a potential performance issue at scales, think thousands of jobs rather than hundreds. Its also not the only cuplrit. ALTER TABLE with structural changes have a similar effect. CREATE OR REPLACE TABLE is ubiquitous in data ingestion pipelines and it doesn’t start to cause a noticeable issue until is deeply ingrained in your developer’s muscle memory. There are alternatives, though.

    Summary of Alternatives

    There are different techniques you can use that will not invalidate the plan cache.

    • Use CREATE TABLE IF NOT EXISTS +  INSERT OVERWRITE is probably my first choice because there is a straight code migration path.
    CREATE TABLE IF NOT EXISTS catalog.schema.table (
id INT,
name STRING
) USING DELTA;
INSERT OVERWRITE catalog.schema.table
SELECT * FROM staging_table;
    • Both MERGE INTO and  COPY INTO have the metadata advantages of the prior solution and support schema evolution as well as concurrency-safe ingestion.
    MERGE INTO catalog.schema.table t
USING (SELECT * FROM staging_table) s
ON t.id = s.id
WHEN MATCHED THEN UPDATE SET *
WHEN NOT MATCHED THEN INSERT *;
    COPY INTO catalog.schema.table
FROM '/mnt/source/'
FILEFORMAT = PARQUET
FORMAT_OPTIONS ('mergeSchema' = 'true');
    • Consider whether you need to be persisting the data beyond the life of the job. If not, consider temporary views or tables. This will avoid Unity Catalog entirely as there is no metadata overhead.
    df.createOrReplaceTempView("job_tmp_view")
    • While I prefer Unity Catalog to handle partitioning strategies in the Silver and Gold layer, you can implement a partitioning scheme with your ingestion logic to keep the metadata stable. This is helpful for high-concurrency workloads.
    CREATE TABLE IF NOT EXISTS catalog.schema.import_data (
id STRING,
source STRING,
load_date DATE
) PARTITIONED BY (source, load_date);
INSERT INTO catalog.schema.import_data
PARTITION (source = 'job_xyz', load_date = current_date())
SELECT * FROM staging;

    I have summarized the different techniques you can use to minimize plan invalidation. In general, I think INSER OVERWRITE usually works well as a drop-in replacement. You get schema evolution with MERGE INTO and COPY INTO. I am often surprised at how many tables that should be considered temporary are stored. This is just a good exercise to go through with your jobs. Finally, there are occasions when the Partition + INSERT paradigm is preferable to INSERT OVERWRITE, particularly for high-concurrency workloads.

    Technique Metadata Cost Plan Invalidation Concurrency-Safe Schema Evolution Notes
    CREATE OR REPLACE TABLE High Yes No Yes Use with caution in production
    INSERT OVERWRITE Low No Yes No Fast for full refreshes
    MERGE INTO Medium No Yes Yes Ideal for idempotent loads
    COPY INTO Low No Yes Yes Great with Auto Loader
    TEMP VIEW / TEMP TABLE None No Yes N/A Best for intermittent pipeline stages
    Partition + INSERT Low No Yes No Efficient for batch-style jobs

    Conclusion

    Tuning the performance characteristics of a platform is more complex than single-application performance tuning. Distributed performance is even more complicated at scale, sice strategies and patterns may start to break down as volume and velocity increase.

    Contact us to learn more about how to empower your teams with the right tools, processes, and training to unlock Databricks’ full potential across your enterprise.

    Source: Read More 

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