Smooth Scan: robust access path selection without cardinality estimation

Abstract

Query optimizers depend heavily on statistics representing column distributions to create good query plans. In many cases, though, statistics are outdated or nonexistent, and the process of refreshing statistics is very expensive, especially for ad hoc workloads on ever bigger data. This results in suboptimal plans that severely hurt performance. The core of the problem is the fixed decision on the type of physical operators that comprise a query plan. This paper makes a case for continuous adaptation and morphing of physical operators throughout their lifetime, by adjusting their behavior in accordance with the observed statistical properties of the data at run time. We demonstrate the benefits of the new paradigm by designing and implementing an adaptive access path operator called Smooth Scan, which morphs continuously within the space of index access and full table scan. Smooth Scan behaves similarly to an index scan for low selectivity; if selectivity increases, however, Smooth Scan progressively morphs its behavior toward a sequential scan. As a result, a system with Smooth Scan requires no optimization decisions on the access paths up front. Additionally, by depending only on the result distribution and eschewing statistics and cardinality estimates altogether, Smooth Scan ensures repeatable execution across multiple query invocations. Smooth Scan implemented in PostgreSQL demonstrates robust, near-optimal performance on micro-benchmarks and real-life workloads, while being statistics oblivious at the same time.