Batch Mode Memory Fractions

Joe Obbish explains what memory fractions are and how incorrect calculations can lead to tempdb spills:

There’s very little information out there about memory fractions. I would define them as information in the query plan that can give you clues about each operator’s share of the total query memory grant. This is naturally more complicated for query plans that insert into tables with columnstore indexes but that won’t be covered here. Most references will tell you not to worry about memory fractions or that they aren’t useful most of the time. Out of thousands of queries that I’ve tuned I can only think of a few for which memory fractions were relevant. Sometimes queries spill to tempdb even though SQL Server reports that a lot of query memory was unused. In these situations I generally hope for a poor cardinality estimate which leads to a memory fraction which is too low for the spilling operator. If fixing the cardinality estimate doesn’t prevent the spill then things can get a lot more complicated, assuming that you don’t just give up.

Extremely interesting post.

Related Posts

Batch Mode Normalization

Paul White digs into batch mode normalization and its consequences for performance: I mentioned in the introduction that not all eight-byte data types can fit in 64 bits. This fact is important because many columnstore and batch mode performance optimizations only work with data 64 bits in size. Aggregate pushdown is one of those things. There are […]

Read More

Comparing CAST and CONVERT Performance

Max Vernon runs a performance test of CAST versus CONVERT: This post is a follow-up to my prior post inspecting the performance of PARSE vs CAST & CONVERT, where we see that PARSE is an order of magnitude slower than CONVERT. In this post, we’ll check if there is a similar difference between using CAST or CONVERT. But just to be clear, CONVERT offers […]

Read More

Categories

February 2018
MTWTFSS
« Jan Mar »
 1234
567891011
12131415161718
19202122232425
262728