Disabling for Index of Azure Synapse Dedicated sql pool

Question

Hi,

I am having an Azure synapse dedicated sql pool instance but wanted to disable the default index creation as part creating the tables. Please let me know all steps around doing that.

Also let me know how much better performance and storage do we get when using CCI vs Heap tables ? example like 10X better or 100 times better etc ?

Thanks

Answer 1

Hi ,

Thanks for reaching out to Microsoft Q&A.

By default in Synapse dedicated SQL pool, if you do not specify an index type when creating a table, it will typically create a clustered columnstore index. To prevent that, you must explicitly state you want a HEAP (or a different index type). Or, if you want to use hash distribution but still have no index.

• HEAP: Instructs Synapse to create the table without any index (i.e., a heap).
• DISTRIBUTION: Specifies how data is distributed (HASH(column_name), ROUND_ROBIN, or REPLICATE).

Note: There is no global switch to turn off the default index creation; you must specify “HEAP” (or another index type) explicitly in each CREATE TABLE statement.

Performance and Storage: CCI vs. Heap

a) Clustered Columnstore Index (CCI)

• Compression: Often 5–10x (sometimes even more) compared to rowstore/heap tables.
• Query Performance: Can be significantly faster (10–100x) for analytical queries that scan large volumes of data, especially with aggregations across many columns.
• Recommended Use: Best for large fact tables in data warehouse scenarios where you primarily run read heavy analytics/aggregations.

b) Heap Tables

• No Automatic Compression:
  • Data is simply stored in pages without an index, so you don’t typically get the compression benefits.
• Faster Write-Load in Some Cases:
  • Loads can be slightly faster for staging or transient tables because no index maintenance is required during insert.
• Less Efficient for Analytics:
  • Queries scanning large heap tables can become slower unless you create additional indexes or ultimately convert them to columnstore for analytical workloads.

Rule of Thumb:

• Storage Savings from CCI: ~5–10x compression vs. rowstore/heap in many real-world scenarios.
• Query Speedup with CCI: Commonly 10x faster for scan-heavy, aggregation-focused queries, and it can go above 50x or 100x in certain extreme cases.

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3. Practical Tips
4. Load into a Heap, Then Convert:
   1. For large fact tables where ingestion speed matters, you can load data into a heap quickly, then create a clustered columnstore index afterward. This can be more efficient than maintaining columnstore during the initial bulk load.
5. Use CCI for Analytics:
   1. For long-term analytical tables, leverage clustered columnstore unless you have a specific reason (unusual query patterns or smaller reference tables) to use a heap or clustered index.
6. Experiment and Measure:
   1. Performance gains vary with data volume, cardinality, and query patterns. Always measure your specific workload in a test environment.

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