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Thursday 29 July 2010

Inside the Optimizer: Constructing a Plan - Part 1

Inside the Optimizer: Constructing a Plan - Part 1

For today’s entry, I thought we might take a look at how the optimizer builds an executable plan using rules. To illustrate the process performed by the optimizer, we will configure it to produce incrementally better plans by progressively applying the necessary rules.

Example Query

Here’s a simple query (using the AdventureWorks sample database) that shows the total number of items in the warehouse for each of a small number of products:

    total_qty = SUM(I.Quantity)
FROM Production.Product P 
JOIN Production.ProductInventory AS I
    ON I.ProductID = P.ProductID
    P.ProductNumber LIKE N'T%'

Heaps of work is done by SQL Server to parse and bind the query before it even reaches the query optimizer, but when it does, it arrives as a tree of logical relational operators, like this:

The optimizer needs to translate this logical tree into a plan that can be executed by the Query Processor. If the query optimizer did nothing more than translate the logical relational operators to the first valid form it found, we would get a plan like this:

This executable plan features two full table scans, a Cartesian Product, a Filter on the two predicates, and an Aggregate. This is a long way from being an optimal query plan, but it does produce correct results.

In case you are wondering, the Compute Scalar is there to ensure that the SUM aggregate returns NULL instead of zero when no rows are processed.

Matching and Applying Rules

The optimizer found that basic plan by replacing logical operations with one of the physical alternatives it knows about. This type of transformation is performed by an implementation rule within the optimizer.

For example, the logical operation “Inner Join” was physically implemented by Nested Loops (other implementation rules exist for Merge and Hash join).

To get from the logical tree to the executable plan shown, a total of five implementation rules were successfully matched and run by the optimiser:

  1. GET to Scan
  2. JOIN to Nested Loops
  3. SELECT to Filter
  4. Group By Aggregate to Stream Aggregate
  5. Group By Aggregate to Hash Aggregate

The first rule replaces the logical GETs with table scans. The second rule implements the logical JOIN using Nested Loops as mentioned before. The third converts all the predicates (including the join predicate) into a Filter operator. The fourth and fifth rules represent two alternative strategies to physically perform the aggregation. In this case, the optimizer chose a Stream Aggregate over a Hash Aggregate, based on costing estimates.

Ok, but no-one would buy SQL Server if it produced that sort of plan on a regular basis. Luckily, there are many other rules that the optimizer can use; in fact, there are nearly four hundred in total. Rest assured that the query optimizer fought hard to resist producing such a dismal plan — more than one dozen separate rules had to be turned off to force it to do so.

As well as more implementation rules, there are a number of exploration and substitution rules. These transform some part of the logical request into an equivalent form, based on mathematical equivalences or heuristics. A simple example of an exploration rule is Join Commute. This rule exploits the fact that A JOIN B is the same as B JOIN A for an inner join.

The optimizer also has simplification rules and enforcer rules. Applying these rules generates alternative strategies, the best of which are incorporated in the final plan. As an aside, a single simplification rule was responsible for the transformation shown in my Segment Top post. That amazing transformation goes by the understated name “Generate Group By Apply”.

Improving the Plan

An obvious deficiency in the basic executable plan above is that it performs a Cartesian product of the two source tables followed by a Filter. It would be much more efficient to evaluate the join predicate at the same time as performing the physical join.

The rule that performs this task is called SELonJN (SELECT on JOIN). I should stress that this is not the T-SQL SELECT statement, it is the relational SELECT operation: a filter applied to rows. By enabling the SELonJN optimisation rule, we get an incrementally better plan:

The Cartesian Product has been replaced by a more normal-looking Nested Loops operator, which is a result of the join predicate being moved from the Filter into the join.

In fact, the Filter operator has disappeared completely. But where has the non-join predicate ProductNumber LIKE 'T%' gone? It has been pushed further down the plan, all the way to the Product table scan, where it appears as a residual (non-seek) predicate.

That is still not a great plan. We are scanning the whole Inventory table once for every row from the Product table that matches on the ProductNumber predicate.

To progress forward, we need rules that know about nonclustered indexes. The basic rule we have relied on so far simply translates a GET to a table scan. Enabling a couple more rules produces:

That’s a little better. We are now using the correct nonclustered indexes, but the predicates are being applied to a scan, not as the index seek we might have hoped for. There are quite a few rules left to apply before we get a really efficient plan. I’ll cover those next time.

This post is part of a series: Part1 | Part 2 | Part3 | Part4

© Paul White
email: SQLkiwi@gmail.com
twitter: @SQL_Kiwi

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