Optimization: Defer Projections for Server Queries #2676
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There are two orthogonal adjustments we can make to avoid extra work for small queries, basically mark the calling contexts when we want to defer projections, mark when we see a default expression that invalidates deferring, mark the top-level projection (not in a subquery/union) in planbuilder when the positive flags are set and no negative flags are set. I think this extra control logic should be similar complexity to switching on the max1RowIter case
sql/analyzer/optimization_rules.go
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| // deferProjections marks projection nodes that can defer projections to the end of the query execution. | ||
| // This can avoid an unnecessary slice allocation specifically in the case where we are spooling rows | ||
| // from the server to the client. | ||
| func deferProjections(ctx *sql.Context, a *Analyzer, node sql.Node, scope *plan.Scope, sel RuleSelector, qFlags *sql.QueryFlags) (sql.Node, transform.TreeIdentity, error) { |
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we can inline this during binding/in planbuilder when we build a default expression
server/handler.go
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| @@ -511,6 +512,24 @@ func resultForEmptyIter(ctx *sql.Context, iter sql.RowIter, resultFields []*quer | |||
| return &sqltypes.Result{Fields: resultFields}, nil | |||
| } | |||
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| // GetDeferredProjections looks for a top-level deferred projection, marks it as deferred, and retrieves its projections | |||
| func GetDeferredProjections(iter sql.RowIter) []sql.Expression { | |||
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the alternative to this tagging queryFlags with a doProjectionDefer in the handler context and having planbuilder mark the defer for the top-level projection, which you identify with flags.IsSet(DoProjectionDefer) && notInSubquery() when building final projection
This PR speeds up spooling queries from the server to client when there is a top level projection.
Changes include:
sql.RowAdditionally, this PR cleans up and refactors some code surrounding projections.