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Joe Chang

Azure database_id inconsistencies

I am working on improvements to my SQL ExecStats tool, including Azure support. Previously, I had put try catch blocks around everything that bombs in Azure. Now I am putting IF EXISTS tests around SQL referencing system views and procedures not in Azure an other. One of issues I encountered is that there is an inconsistency between DB_ID() and sys.databases.

SELECT database_id FROM sys.databases WHERE name = DB_NAME()

On my Azure database, I have just one user database. We would normally expect that the first user database has database_id = 5.

And indeed SELECT DB_ID() does produce 5, but sys.databases shows database_id 1 for master, and 4 for my user database.

The reason this is annoying is because ExecStats attempts to correlate index usage to execution plans, I need consistent values to be reported by each of many queries. Most of the ExecStats code has already replaced DB_ID(), with a query to sys.databases WHERE name = DB_NAME().

But sys.dm_db_index_usage_stats reports the user database as having database_id = 5.

So for now, I am just subtracting 1 from the database_id in sys.dm_db_index_usage_stats but I have not tested this in an Azure instance with more than 1 user DB.

By the way, the latest ExecStats is today, 2017-05-23 2017-05-25

Edit, note date change above

Edit 2017-06-4

Same is true of dm_exec_sql_text, dm_exec_text_query_plan and probably more

Published Tuesday, May 23, 2017 5:11 PM by jchang

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Razvan Socol said:

This seems to be fixed. Can you check again?

September 13, 2018 12:48 AM

Razvan Socol said:

I see now that this is "documented" to be unreliable: in a Microsoft employee wrote:

"One shouldn't rely on database id / db_id bultin in azure as it changes whenever database moves to a different sql instance (because of the database failover). Use name instead of the id. Also please note that the db_id can potentially be different than in the sys.databases"

September 13, 2018 1:04 AM

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October 22, 2018 11:54 PM

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About jchang

Reverse engineering the SQL Server Cost Based Optimizer (Query Optimizer), NUMA System Architecture, performance tools developer - SQL ExecStats, mucking with the data distribution statistics histogram - decoding STATS_STREAM, Parallel Execution plans, microprocessors, SSD, HDD, SAN, storage performance, performance modeling and prediction, database architecture, SQL Server engine

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