网站优化怎么做 有什么技巧,wordpress被和谐,重庆网站seo好不好,广东短视频推广公司在没有pg_class的时候#xff0c;数据库怎么访问系统表#xff1f;这个问题可以分成两个阶段来看#xff1a;
数据库簇初始化#xff0c;此时一个database都没有#xff0c;所以怎么构造和访问pg_class等系统表是一个问题私有内存初始化系统表。PG的系统表信息是放在back…在没有pg_class的时候数据库怎么访问系统表这个问题可以分成两个阶段来看
数据库簇初始化此时一个database都没有所以怎么构造和访问pg_class等系统表是一个问题私有内存初始化系统表。PG的系统表信息是放在backend本地进程上的backend在初始化的时候又怎么load pg_class
初始化数据字典
在数据库还没有初始化的时候明显是不能通过访问数据字典来初始化database、pg_class等等对象的因为一个库都没有就不能create database也没有pg_class去查元数据信息。 PG通过bki文件的特殊语言初始化一些数据结构然后在bootstrap模式初始化一个原始database1。
编译阶段genbki.h genbki.pl
src/include/catalog/genbki.h * genbki.h defines CATALOG(), BKI_BOOTSTRAP and related macros* so that the catalog header files can be read by the C compiler.* (These same words are recognized by genbki.pl to build the BKI* bootstrap file from these header files.)genbki.h内容很少主要是为了catalog相关操作的宏定义以及给KBI bootstrap文件的宏定义。数据字典的头文件基本都包含genbki.h genbki.pl会在编译过程读取/src/include/catalog目录下的.h表定义文件不含pg_*_d.h并创建postgres.bki文件和pg_*_d.h头文件。 以pg_class为例
[postgrescatalog]$ ll |grep pg_class
-rw-r----- 1 postgres postgres 3682 Aug 6 2019 pg_class.dat
lrwxrwxrwx 1 postgres postgres 86 Apr 8 20:31 pg_class_d.h - /lzl/soft/postgresql-11.5/src/backend/catalog/pg_class_d.h
-rw-r----- 1 postgres postgres 5219 Aug 6 2019 pg_class.hpg_*_d.h头文件就是genbki.pl生成的。pg_*_d.h文件中都包含下面的一段话 It has been GENERATED by src/backend/catalog/genbki.pl 每个数据字典都有一个结构体typedef struct FormData_*catalogname*用以存储数据字典的行数据2例如pg_class的FormData_pg_class
CATALOG(pg_class,1259,RelationRelationId) BKI_BOOTSTRAP BKI_ROWTYPE_OID(83,RelationRelation_Rowtype_Id) BKI_SCHEMA_MACRO
{/* oid */Oid oid;/* class name */NameData relname;/* OID of namespace containing this class */Oid relnamespace BKI_DEFAULT(pg_catalog) BKI_LOOKUP(pg_namespace);/* OID of entry in pg_type for relations implicit row type, if any */Oid reltype BKI_LOOKUP_OPT(pg_type);/* OID of entry in pg_type for underlying composite type, if any */Oid reloftype BKI_DEFAULT(0) BKI_LOOKUP_OPT(pg_type);/* class owner */Oid relowner BKI_DEFAULT(POSTGRES) BKI_LOOKUP(pg_authid);.../* access-method-specific options */text reloptions[1] BKI_DEFAULT(_null_);/* partition bound node tree */pg_node_tree relpartbound BKI_DEFAULT(_null_);
#endif
} FormData_pg_class;
pg_class的OID写死了1259所有字段都在FormData_pg_class结构体中。 用户存储数据的结构体初始化后会使用对应的.dat文件插入基础数据。pg_class中会插入4条数据可以理解为bootstrap itempg15中的数据字典表有49个
{ oid 1247,relname pg_type, reltype pg_type },
{ oid 1249,relname pg_attribute, reltype pg_attribute },
{ oid 1255,relname pg_proc, reltype pg_proc },
{ oid 1259,relname pg_class, reltype pg_class },postgres# select oid,relname from pg_class where oid::int 1247 and oid::int1259;oid | relname
--------------------1247 | pg_type1249 | pg_attribute1255 | pg_proc1259 | pg_class把基础数据字典写入后其他的都可以依赖这些数据生成。
初始化database阶段initdbpostgres.bki
initdb.c中的注释 * To create template1, we run the postgres (backend) program in bootstrap* mode and feed it data from the postgres.bki library file. After this* initial bootstrap phase, some additional stuff is created by normal* SQL commands fed to a standalone backend. 以bootstrap模式启动backend并运行postgres.bki脚本postgres.bki可以在没有任何系统表的情况下执行相关函数。此后才可以使用正常的SQL文件和启动标准的backend进程。 template1可以称之为bootstrap database了postgres、template0两个库是在template1建立以后才创建
void
initialize_data_directory(void)
{
.../* Bootstrap template1 */bootstrap_template1();
...make_template0(cmdfd);make_postgres(cmdfd);PG_CMD_CLOSE;check_ok();
}有了template1后make_template0和make_postgres创建对应的template0 database和postgres database直接用一般的SQL语句CREATE DATABASE命令创建
/** copy template1 to postgres*/
static void
make_postgres(FILE *cmdfd)
{const char *const *line;/** Just as we did for template0, and for the same reasons, assign a fixed* OID to postgres and select the file_copy strategy.*/static const char *const postgres_setup[] {CREATE DATABASE postgres OID CppAsString2(PostgresDbOid) STRATEGY file_copy;\n\n,COMMENT ON DATABASE postgres IS default administrative connection database;\n\n,NULL};for (line postgres_setup; *line; line)PG_CMD_PUTS(*line);
}backend本地缓存数据字典
PG私有内存的基础知识可参考PostgreSQL内存浅析3。
PG的数据字典信息存放在本地backend进程中非共享。数据字典缓存主要关注的是syscache/catcache和relcache他们分别缓存系统表和表模式信息。 其中syscache/catcache是用于缓存系统表的syscache相当于catcache的上层结构。syscache是一个数组数字中的每个元素对应一个catcache每个catcache对应一个系统表1。
//PG15.3 SysCacheSize35
static CatCache *SysCache[SysCacheSize];pg在fork backend的时候调用的是InitPostgres其中会调用syscache/catcache和relcache的初始化函数。下面来看看backend的初始化。
syscache/catcache初始化
struct cachedesc
{Oid reloid; /* OID of the relation being cached */Oid indoid; /* OID of index relation for this cache */int nkeys; /* # of keys needed for cache lookup */int key[4]; /* attribute numbers of key attrs */int nbuckets; /* number of hash buckets for this cache */
};static const struct cachedesc cacheinfo[] {{
... {RelationRelationId, /* RELNAMENSP */ClassNameNspIndexId,2,{Anum_pg_class_relname,Anum_pg_class_relnamespace,0,0},128},{RelationRelationId, /* RELOID */ClassOidIndexId,1,{Anum_pg_class_oid,0,0,0},128
...
};例如pg_class由genbki.pl生成的pg_class_d.h中定义Anum_pg_class_oid
#define Anum_pg_class_oid 1reloid就是oid select oid,relname from pg_class where oid::int 1247 and oid::int1259;oid | relname
--------------------1259 | pg_classInitCatalogCache其实是初始化syscache数组也就是初始化所有的catcache。InitCatalogCache最终通过InitCatCache全量初始化CatCache这里其中一个就有pg_class的)
void
InitCatalogCache(void)
{
...for (cacheId 0; cacheId SysCacheSize; cacheId){SysCache[cacheId] InitCatCache(cacheId,cacheinfo[cacheId].reloid,cacheinfo[cacheId].indoid,cacheinfo[cacheId].nkeys,cacheinfo[cacheId].key,cacheinfo[cacheId].nbuckets);if (!PointerIsValid(SysCache[cacheId]))elog(ERROR, could not initialize cache %u (%d),cacheinfo[cacheId].reloid, cacheId);/* Accumulate data for OID lists, too */SysCacheRelationOid[SysCacheRelationOidSize] cacheinfo[cacheId].reloid;SysCacheSupportingRelOid[SysCacheSupportingRelOidSize] cacheinfo[cacheId].reloid;SysCacheSupportingRelOid[SysCacheSupportingRelOidSize] cacheinfo[cacheId].indoid;/* see comments for RelationInvalidatesSnapshotsOnly */Assert(!RelationInvalidatesSnapshotsOnly(cacheinfo[cacheId].reloid));}
...CacheInitialized true;
}然后来到catcache.c。 InitCatCache会开辟内存并且放到CacheMemoryContext中管理。它也只是把宏定义的一些oid赋值给对应的catcache此时还没有open表
/** InitCatCache** This allocates and initializes a cache for a system catalog relation.* Actually, the cache is only partially initialized to avoid opening the* relation. The relation will be opened and the rest of the cache* structure initialized on the first access.*/
CatCache *
InitCatCache(int id,Oid reloid,Oid indexoid,int nkeys,const int *key,int nbuckets)
{
...oldcxt MemoryContextSwitchTo(CacheMemoryContext);
...sz sizeof(CatCache) PG_CACHE_LINE_SIZE;cp (CatCache *) CACHELINEALIGN(palloc0(sz));cp-cc_bucket palloc0(nbuckets * sizeof(dlist_head));/** initialize the caches relation information for the relation* corresponding to this cache, and initialize some of the new caches* other internal fields. But dont open the relation yet.*/cp-id id;cp-cc_relname (not known yet);cp-cc_reloid reloid;cp-cc_indexoid indexoid;cp-cc_relisshared false; /* temporary */cp-cc_tupdesc (TupleDesc) NULL;cp-cc_ntup 0;cp-cc_nbuckets nbuckets;cp-cc_nkeys nkeys;for (i 0; i nkeys; i)cp-cc_keyno[i] key[i];
...MemoryContextSwitchTo(oldcxt);return cp;
}id是catcache数组元素的编号赋值的reloid是已知的cacheinfo中的oid也赋值了cacheinfo中的key[4]其他信息基本都还不知道例如relname、tupdesc因为到这里系统表还没有open。 catcache只有在search的时候才有open的操作虽然函数名字类似*init*不过已经不在初始化的过程中了相关函数不再这里展示。 syscache/catcache初始化完成后实际上是没有任何元组信息的。
relcache初始化
relcache初始化这篇PostgreSQL内存浅析已经讲的比较好了。 relcache初始化由5个阶段:
RelationCacheInitialize - 初始化relcache初始化为空的RelationCacheInitializePhase2 - 初始化共享的catalog并加载5个global系统表RelationCacheInitializePhase3 - 完成初始化relcache并加载4个基础系统表RelationIdGetRelation - 通过relation id获得rel描述RelationClose - 关闭一个relation
其中RelationCacheInitializePhase2 RelationCacheInitializePhase3 都有load系统表他们有先后顺序的必要。 RelationCacheInitializePhase2有兴趣的可以自行查看函数也load几个系统表RelationCacheInitializePhase3 是与我们的问题相关的我们看这个
/** RelationCacheInitializePhase3** This is called as soon as the catcache and transaction system* are functional and we have determined MyDatabaseId. At this point* we can actually read data from the databases system catalogs.* We first try to read pre-computed relcache entries from the local* relcache init file. If thats missing or broken, make phony entries* for the minimum set of nailed-in-cache relations. Then (unless* bootstrapping) make sure we have entries for the critical system* indexes. Once weve done all this, we have enough infrastructure to* open any system catalog or use any catcache. The last step is to* rewrite the cache files if needed.*/
void
RelationCacheInitializePhase3(void)
{
...if (IsBootstrapProcessingMode() ||!load_relcache_init_file(false)){needNewCacheFile true;formrdesc(pg_class, RelationRelation_Rowtype_Id, false,Natts_pg_class, Desc_pg_class);formrdesc(pg_attribute, AttributeRelation_Rowtype_Id, false,Natts_pg_attribute, Desc_pg_attribute);formrdesc(pg_proc, ProcedureRelation_Rowtype_Id, false,Natts_pg_proc, Desc_pg_proc);formrdesc(pg_type, TypeRelation_Rowtype_Id, false,Natts_pg_type, Desc_pg_type);#define NUM_CRITICAL_LOCAL_RELS 4 /* fix if you change list above */}MemoryContextSwitchTo(oldcxt);/* In bootstrap mode, the faked-up formrdesc info is all well have */if (IsBootstrapProcessingMode())return;.../* now write the files */write_relcache_init_file(true);write_relcache_init_file(false);}
}IsBootstrapProcessingMode其实是专门为bootstrap模式定制的判断一般的backend是不满足这个条件的。 load_relcache_init_file(false)尝试从initfile中加载系统表信息load_relcache_init_file(false)传入的是false表示是私有initfile不是共享initfile
[postgres16384]$ pwd
/pgdata/lzl/data15_6879/base/16384
--粗糙一点看。strings会忽略一部分信息但是表和列名可以看到
[postgres16384]$ strings pg_internal.init |grep pg_class
pg_class_oid_index
pg_class
pg_class_relname_nsp_index
[postgres16384]$ strings pg_internal.init |grep -E pg_class|relname
pg_class_oid_index
pg_class
relname
relnamespace
pg_class_relname_nsp_index
relname
relnamespace如果initfile损坏或者没有那么加载initfile失败进入判断去load 4个基础系统表 //跟2阶段差不多加载更多的系统表描述if (IsBootstrapProcessingMode() ||!load_relcache_init_file(false)){needNewCacheFile true;formrdesc(pg_class, RelationRelation_Rowtype_Id, false,Natts_pg_class, Desc_pg_class);formrdesc(pg_attribute, AttributeRelation_Rowtype_Id, false,Natts_pg_attribute, Desc_pg_attribute);formrdesc(pg_proc, ProcedureRelation_Rowtype_Id, false,Natts_pg_proc, Desc_pg_proc);formrdesc(pg_type, TypeRelation_Rowtype_Id, false,Natts_pg_type, Desc_pg_type);有了pg_class 4个基础表后面加载系统表信息一切都很简单了
References 《PostgreSQL内核分析》第23章 ↩︎ ↩︎ https://www.postgresql.org/docs/current/system-catalog-declarations.html ↩︎ PostgreSQL内存浅析 ↩︎
