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Release memory allocated by dependency_degree

Calculating degree of a functional dependency may allocate a lot ofmemory - we have released mot of the explicitly allocated memory, bute.g. detoasted varlena values were left behind. That may be an issue,because we consider a lot of dependencies (all combinations), and thedetoasting may happen for each one again.Fixed by calling dependency_degree() in a dedicated context, andresetting it after each call. We only need the calculated dependencydegree, so we don't need to copy anything.Backpatch to PostgreSQL 10, where extended statistics were introduced.Backpatch-through: 10Discussion:https://www.postgresql.org/message-id/20210915200928.GP831%40telsasoft.com

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src/backend/statistics
- dependencies.c

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+16

-6

lines changed

`‎src/backend/statistics/dependencies.c`

Lines changed: 16 additions & 6 deletions

Original file line number	Diff line number	Diff line change
`@@ -30,6 +30,7 @@`
`30`	`30`	`#include"utils/fmgroids.h"`
`31`	`31`	`#include"utils/fmgrprotos.h"`
`32`	`32`	`#include"utils/lsyscache.h"`
	`33`	`+#include"utils/memutils.h"`
`33`	`34`	`#include"utils/selfuncs.h"`
`34`	`35`	`#include"utils/syscache.h"`
`35`	`36`	`#include"utils/typcache.h"`
`@@ -325,12 +326,6 @@ dependency_degree(StatsBuildData data, int k, AttrNumber dependency)`
`325`	`326`	`group_size++;`
`326`	`327`	`}`
`327`	`328`
`328`		`-if (items)`
`329`		`-pfree(items);`
`330`		`-`
`331`		`-pfree(mss);`
`332`		`-pfree(attnums_dep);`
`333`		`-`
`334`	`329`	`/* Compute the 'degree of validity' as (supporting/total). */`
`335`	`330`	`return (n_supporting_rows*1.0 /data->numrows);`
`336`	`331`	`}`
`@@ -359,9 +354,15 @@ statext_dependencies_build(StatsBuildData *data)`
`359`	`354`
`360`	`355`	`/* result */`
`361`	`356`	`MVDependencies*dependencies=NULL;`
	`357`	`+MemoryContextcxt;`
`362`	`358`
`363`	`359`	`Assert(data->nattnums >=2);`
`364`	`360`
	`361`	`+/* tracks memory allocated by dependency_degree calls */`
	`362`	`+cxt=AllocSetContextCreate(CurrentMemoryContext,`
	`363`	`+"dependency_degree cxt",`
	`364`	`+ALLOCSET_DEFAULT_SIZES);`
	`365`	`+`
`365`	`366`	`/*`
`366`	`367`	`* We'll try build functional dependencies starting from the smallest ones`
`367`	`368`	`* covering just 2 columns, to the largest ones, covering all columns`
`@@ -380,10 +381,17 @@ statext_dependencies_build(StatsBuildData *data)`
`380`	`381`	`{`
`381`	`382`	`doubledegree;`
`382`	`383`	`MVDependency*d;`
	`384`	`+MemoryContextoldcxt;`
	`385`	`+`
	`386`	`+/* release memory used by dependency degree calculation */`
	`387`	`+oldcxt=MemoryContextSwitchTo(cxt);`
`383`	`388`
`384`	`389`	`/* compute how valid the dependency seems */`
`385`	`390`	`degree=dependency_degree(data,k,dependency);`
`386`	`391`
	`392`	`+MemoryContextSwitchTo(oldcxt);`
	`393`	`+MemoryContextReset(cxt);`
	`394`	`+`
`387`	`395`	`/*`
`388`	`396`	`* if the dependency seems entirely invalid, don't store it`
`389`	`397`	`*/`
`@@ -425,6 +433,8 @@ statext_dependencies_build(StatsBuildData *data)`
`425`	`433`	`DependencyGenerator_free(DependencyGenerator);`
`426`	`434`	`}`
`427`	`435`
	`436`	`+MemoryContextDelete(cxt);`
	`437`	`+`
`428`	`438`	`returndependencies;`
`429`	`439`	`}`
`430`	`440`

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`‎src/backend/statistics/dependencies.c`

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