plan, tidb: support plan cache for SELECT statement #4644
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@zz-jason Good job! |
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@shenli WIP, benchmark result will be provided later. |
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When transaction changed from read only to not read only. The plan will be changed since dirty table exists. |
2. add cacheable checker
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Please fix CI @zz-jason |
executor/compiler.go
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| p, err := plan.Optimize(ctx, node, is) | ||
| if err != nil { | ||
| return nil, errors.Trace(err) | ||
| } | ||
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| _, isDual := p.(*plan.TableDual) | ||
| if isDual { | ||
| cacheable = false |
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create table t(a bigint);
select * from t where exists (select * from t);
this query will be optimized to a TableDual
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@coocood PTAL |
plan/cache/simple_lru_test.go
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| c.Assert(lru.size, Equals, lru.capacity) | ||
| c.Assert(lru.size, Equals, int64(3)) | ||
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| // test for nonexistence elements |
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s/ nonexistence/ non-existent
plan/cache/simple_lru_test.go
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| c.Assert(element, IsNil) | ||
| } | ||
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| // test for in existence elements |
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s/ in existence/ existent
plan/cache/simple_lru_test.go
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| lru.Put(keys[i], vals[i]) | ||
| } | ||
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| // test for nonexistence elements |
plan/cacheable_checker.go
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| switch expr := exprNode.(type) { | ||
| case *ast.VariableExpr, *ast.ExistsSubqueryExpr: | ||
| return false | ||
| case *ast.UnaryOperationExpr: |
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This is not needed, V will be visited.
plan/cacheable_checker.go
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| // Enter implements Visitor interface. | ||
| func (checker *cacheableChecker) Enter(in ast.Node) (out ast.Node, skipChildren bool) { | ||
| switch node := in.(type) { | ||
| case *ast.SelectStmt: |
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We don't need to do this, every expression in SelectStmt will be visited.
plan/cacheable_checker.go
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| } | ||
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| // Leave implements Visitor interface. | ||
| func (checker *cacheableChecker) Leave(in ast.Node) (out ast.Node, skipChildren bool) { |
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Just switch the type of in here will do.
executor/compiler.go
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| if err := plan.Preprocess(node, is, ctx); err != nil { | ||
| return nil, errors.Trace(err) | ||
| // Compile compiles an ast.StmtNode to a physical plan. | ||
| func (c *Compiler) Compile(ctx context.Context, node ast.StmtNode) (is infoschema.InfoSchema, p plan.Plan, expensive bool, cacheable bool, err error) { |
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This function has too many return values, how about just return ExecStmt?
session.go
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| // Step3: Cache the physical plan if possiable. | ||
| if cache.PlanCacheEnabled && cacheable && len(stmtNodes) == 1 { | ||
| if _, isSelect := stmtNodes[0].(*ast.SelectStmt); isSelect { | ||
| cache.GlobalPlanCache.Put(cacheKey, cache.NewSQLCacheValue(stmtNode, plan, expensive)) |
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isSelect is covered by cacheable already.
plan/cache/sharded_lru.go
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| // PlanCacheEnabled stores the global config "plan-cache-enabled". | ||
| PlanCacheEnabled bool | ||
| // PlanCacheShards stores the global config "plan-cache-shards". | ||
| PlanCacheShards int64 = 200 |
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Use a power of 2 value is more efficient.
plan/cache/key.go
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| func (key *sqlCacheKey) Hash() []byte { | ||
| if key.hash == nil { | ||
| var ( | ||
| userBytes = []byte(key.user) |
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We can use hack.Slice to avoid memory allocation.
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/run-all-test |
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LGTM |
| ) | ||
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| // Cacheable checks whether the input ast is cacheable. | ||
| func Cacheable(node ast.Node) bool { |
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Need some test for this function.
plan/cacheable_checker.go
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| // Enter implements Visitor interface. | ||
| func (checker *cacheableChecker) Enter(in ast.Node) (out ast.Node, skipChildren bool) { | ||
| switch in.(type) { | ||
| case *ast.VariableExpr, *ast.ExistsSubqueryExpr: |
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Query contains non-deterministic function like Now(), RAND() should not be cacheable.
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done. btw, rand() can be cached
session.go
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| } | ||
| sessionExecuteCompileDuration.Observe(time.Since(startTS).Seconds()) | ||
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| // Step3: Cache the physical plan if possiable. |
config/config.go
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| @@ -121,6 +129,11 @@ var defaultConf = Config{ | |||
| XHost: "0.0.0.0", | |||
| XPort: 14000, | |||
| }, | |||
| PlanCache: PlanCache{ | |||
| Enabled: true, | |||
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Set default to true is not safe for now.
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only for the ci test, will be disabled in the last commit of this pr
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LGTM |
main change:
func (s *session) Execute(sql string) ([]ast.RecordSet, error), we check the opportunity to use plan cache first