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Optimize roles_is_member_of() with a Bloom filter.

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When the list of roles gathered by roles_is_member_of() grows very
large, a Bloom filter is created to help avoid some linear searches
through the list.  The threshold for creating the Bloom filter is
set arbitrarily high and may require future adjustment.

Suggested-by: Tom Lane
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/CAGvXd3OSMbJQwOSc-Tq-Ro1CAz%3DvggErdSG7pv2s6vmmTOLJSg%40mail.gmail.com
This commit is contained in:
Nathan Bossart 2024-03-26 14:43:37 -05:00
parent fad3b5b5ac
commit d365ae7054
1 changed files with 65 additions and 3 deletions
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68
src/backend/utils/adt/acl.c
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@ -36,6 +36,7 @@
#include "common/hashfn.h"
#include "foreign/foreign.h"
#include "funcapi.h"
#include "lib/bloomfilter.h"
#include "lib/qunique.h"
#include "miscadmin.h"
#include "utils/acl.h"
@ -78,6 +79,13 @@ static Oid cached_role[] = {InvalidOid, InvalidOid, InvalidOid};
static List *cached_roles[] = {NIL, NIL, NIL};
static uint32 cached_db_hash;
/*
* If the list of roles gathered by roles_is_member_of() grows larger than the
* below threshold, a Bloom filter is created to speed up list membership
* checks. This threshold is set arbitrarily high to avoid the overhead of
* creating the Bloom filter until it seems likely to provide a net benefit.
*/
#define ROLES_LIST_BLOOM_THRESHOLD 1024
static const char *getid(const char *s, char *n, Node *escontext);
static void putid(char *p, const char *s);
@ -4918,6 +4926,53 @@ RoleMembershipCacheCallback(Datum arg, int cacheid, uint32 hashvalue)
cached_role[ROLERECURSE_SETROLE] = InvalidOid;
}
/*
* A helper function for roles_is_member_of() that provides an optimized
* implementation of list_append_unique_oid() via a Bloom filter. The caller
* (i.e., roles_is_member_of()) is responsible for freeing bf once it is done
* using this function.
*/
static inline List *
roles_list_append(List *roles_list, bloom_filter **bf, Oid role)
{
unsigned char *roleptr = (unsigned char *) &role;
/*
* If there is a previously-created Bloom filter, use it to try to
* determine whether the role is missing from the list. If it says yes,
* that's a hard fact and we can go ahead and add the role. If it says
* no, that's only probabilistic and we'd better search the list. Without
* a filter, we must always do an ordinary linear search through the
* existing list.
*/
if ((*bf && bloom_lacks_element(*bf, roleptr, sizeof(Oid))) ||
!list_member_oid(roles_list, role))
{
/*
* If the list is large, we take on the overhead of creating and
* populating a Bloom filter to speed up future calls to this
* function.
*/
if (*bf == NULL &&
list_length(roles_list) > ROLES_LIST_BLOOM_THRESHOLD)
{
*bf = bloom_create(ROLES_LIST_BLOOM_THRESHOLD * 10, work_mem, 0);
foreach_oid(roleid, roles_list)
bloom_add_element(*bf, (unsigned char *) &roleid, sizeof(Oid));
}
/*
* Finally, add the role to the list and the Bloom filter, if it
* exists.
*/
roles_list = lappend_oid(roles_list, role);
if (*bf)
bloom_add_element(*bf, roleptr, sizeof(Oid));
}
return roles_list;
}
/*
* Get a list of roles that the specified roleid is a member of
*
@ -4946,6 +5001,7 @@ roles_is_member_of(Oid roleid, enum RoleRecurseType type,
ListCell *l;
List *new_cached_roles;
MemoryContext oldctx;
bloom_filter *bf = NULL;
Assert(OidIsValid(admin_of) == PointerIsValid(admin_role));
if (admin_role != NULL)
@ -5023,16 +5079,22 @@ roles_is_member_of(Oid roleid, enum RoleRecurseType type,
* graph, we must test for having already seen this role. It is
* legal for instance to have both A->B and A->C->B.
*/
roles_list = list_append_unique_oid(roles_list, otherid);
roles_list = roles_list_append(roles_list, &bf, otherid);
}
ReleaseSysCacheList(memlist);
/* implement pg_database_owner implicit membership */
if (memberid == dba && OidIsValid(dba))
roles_list = list_append_unique_oid(roles_list,
ROLE_PG_DATABASE_OWNER);
roles_list = roles_list_append(roles_list, &bf,
ROLE_PG_DATABASE_OWNER);
}
/*
* Free the Bloom filter created by roles_list_append(), if there is one.
*/
if (bf)
bloom_free(bf);
/*
* Copy the completed list into TopMemoryContext so it will persist.
*/