1先上个图，看一下函数调用过程梗概，中间细节有略
GUC参数初始化分两步，第一步先读取buildin/compiled-in的GUC参数默认值，这里包括全部的GUC参数，建立GUC参数相关结构变量，第二步读取postgresql.conf配置文件中的参数设置之。从上图中能看出来，这个读取并设置postgresql.conf中参数的过程还是挺复杂的。
2初始化GUC相关数据结构并取hardcode/buildin的参数值。
pg里的GUC参数按设置的值分五种类型，分别是bool、int、real、string、enum，根据这五种类型，定义了五种结构类型，再根据这五种结构，每个类型建一个对应的静态数组，用于存储这些相应类型的GUC参数。这五种类型是config_bool、config_int、config_real、config_string、config_enum，对应的静态数组是ConfigureNamesBool、ConfigureNamesInt、ConfigureNamesReal、ConfigureNamesString、ConfigureNamesEnum。具体结构和数组定义见下面。
五个结构定义：
struct config_bool
{
struct config_generic gen;
/* these fields must be set correctly in initial value: */
/* (all but reset_val are constants) */
bool *variable;
bool boot_val;
GucIntCheckHookcheck_hook;
GucBoolAssignHookassign_hook;
GucShowHookshow_hook;
/* variable fields, initialized at runtime: */
bool reset_val;
void * reset_extra
};
struct config_int
{
struct config_generic gen;
/*constant fields, must be set correctly in initial value: */
int *variable;
int boot_val;
int min;
int max;
GucIntCheckHookcheck_hook;
GucIntAssignHookassign_hook;
GucShowHookshow_hook;
/* variable fields, initialized at runtime: */
int reset_val;
void * reset_extra
};
struct config_real
{
struct config_generic gen;
/* constantfields, must be set correctly in initial value: */
double *variable;
double boot_val;
double min;
double max;
GucIntCheckHookcheck_hook;
GucRealAssignHookassign_hook;
GucShowHookshow_hook;
/* variable fields, initialized at runtime: */
double reset_val;
void * reset_extra
};
struct config_string
{
struct config_generic gen;
/* constant fields, must beset correctly in initial value: */
char **variable;
const char *boot_val;
GucIntCheckHookcheck_hook;
GucStringAssignHookassign_hook;
GucShowHookshow_hook;
/* variable fields, initialized at runtime: */
char *reset_val;
void * reset_extra
};
struct config_enum
{
struct config_generic gen;
/* constant fields, must beset correctly in initial value: */
int *variable;
int boot_val;
GucIntCheckHookcheck_hook;
GucStringAssignHookassign_hook;
GucShowHookshow_hook;
/* variable fields, initialized at runtime: */
int reset_val;
void * reset_extra
};
和结构类型对应的五个静态数组：
static struct config_boolConfigureNamesBool[] =
{
{
{"enable_seqscan",PGC_USERSET, QUERY_TUNING_METHOD,
gettext_noop("Enablesthe planner's use of sequential-scan plans."),
NULL
},
&enable_seqscan,
true,
NULL,NULL, NULL
},
……
/*End-of-list marker */
{
{NULL,0, 0, NULL, NULL}, NULL, false, NULL, NULL, NULL
}
};
static struct config_int ConfigureNamesInt[]=
{
{
{"archive_timeout",PGC_SIGHUP, WAL_ARCHIVING,
gettext_noop("Forcesa switch to the next xlog file if a "
"new file has not been started within Nseconds."),
NULL,
GUC_UNIT_S
},
&XLogArchiveTimeout,
0,0, INT_MAX,
NULL,NULL, NULL
},
……
/*End-of-list marker */
{
{NULL,0, 0, NULL, NULL}, NULL, 0, 0, 0, NULL, NULL, NULL
}
};
static struct config_realConfigureNamesReal[] =
{
{
{"seq_page_cost",PGC_USERSET, QUERY_TUNING_COST,
gettext_noop("Setsthe planner's estimate of the cost of a "
"sequentially fetched disk page."),
NULL
},
&seq_page_cost,
DEFAULT_SEQ_PAGE_COST,0, DBL_MAX,
NULL,NULL, NULL
},
/*End-of-list marker */
{
{NULL,0, 0, NULL, NULL}, NULL, 0.0, 0.0, 0.0, NULL, NULL, NULL
}
};
static struct config_stringConfigureNamesString[] =
{
{
{"archive_command",PGC_SIGHUP, WAL_ARCHIVING,
gettext_noop("Setsthe shell command that will be called to archive a WAL file."),
NULL
},
&XLogArchiveCommand,
"",
NULL,NULL, show_archive_command
},
……
/*End-of-list marker */
{
{NULL,0, 0, NULL, NULL}, NULL, NULL, NULL, NULL, NULL
}
};
static struct config_enumConfigureNamesEnum[] =
{
{
{"backslash_quote",PGC_USERSET, COMPAT_OPTIONS_PREVIOUS,
gettext_noop("Setswhether \"\\'\" is allowed in string literals."),
NULL
},
&backslash_quote,
BACKSLASH_QUOTE_SAFE_ENCODING,backslash_quote_options,
NULL,NULL, NULL
},
……
/*End-of-list marker */
{
{NULL,0, 0, NULL, NULL}, NULL, 0, NULL, NULL, NULL, NULL
}
};
上面五个结构定义中，每个结构的第一个成员变量都是一个config_generic结构的gen成员，下面是config_generic的结构定义：
struct config_generic
{
/* constantfields, must be set correctly in initial value: */
const char *name; /* name of variable - MUST BE FIRST */
GucContext context; /* context required to set the variable */
enumconfig_group group; /* to help organize variables by function */
const char *short_desc; /* short desc. of this variable's purpose */
const char *long_desc; /* long desc. of this variable's purpose */
int flags; /* flag bits, see below*/
/* variablefields, initialized at runtime: */
enumconfig_type vartype; /* type of variable (set only at startup) */
int status; /* status bits, see below*/
GucSource reset_source; /* source of thereset_value */
GucSource source; /*source of the current actual value */
GucStack *stack; /* stacked outside-of-transaction states */
void *extra; /*"extra" pointer for current actual value */
char *sourcefile; /* filecurrent setting is from (NULL if not
* file) */
int sourceline; /* line in source file */
};
然后，定义一个config_generic **类型的静态变量数组guc_variables，再计算参数总数，所有参数以config_generic*类型计算所需内存空间，冗余25%内存后malloc分配内存空间。把guc_variables每一个元素指向ConfigureNamesBool、ConfigureNamesInt、ConfigureNamesReal、ConfigureNamesString、ConfigureNamesEnum这五个数组的config_generic类型成员gen的地址，然后按照参数名称把所有元素做了快速排序。这个过程中还设置了一些GUC参数的默认值。
static struct config_generic **guc_variables;
后面查询GUC参数都是在guc_variables这个已排序的数组里找。这样GUC参数的数据结构就搭建完成了，下面看看GUC参数相关的数据结构图吧。
先把涉及到的结构的图分别列出，再画个这些结构的组织关系示意图。
3加载postgresql.conf参数配置文件里的参数设置
从main->PostmasterMain->SelectConfigFiles->ProcessConfigFile开始处理参数配置文件postgresql.conf，读取postgresql.conf配置文件的调用过程是ProcessConfigFile -> ParseConfigFile -> AllocateFile->fopen，最后用fopen打开文件，其中ProcessConfigFile、ParseConfigFile在文件src\backend\utils\misc\guc-file.l中，AllocateFile在文件src\backend\storage\file\fd.c中。
pg使用 flex 去处理 conf 文件。在ParseConfigFile中把配置文件中的配置项组织成一个链表，调用set_config_option检查这些值是否有效，若可以设置就调用set_config_option设置这些值。
这里以"max_connections"做例子，从配置文件读取"max_connections"，然后从guc_variables数组中找元素" max_connections "，比较参数结构的GucContext （枚举类参数能被设置的时机。定义见下面）枚举类型成员context和当前时间，看是否可以此刻修改。接着比较参数结构的GucSource（枚举了当前GUC参数设置的来源。除非参数新值的来源等级不小于原参数的来源等级时，新设置才能生效。例如，修改配置文件不能覆盖postmaster command line的设置。定义见下面）枚举类型成员source和新参数值的来源，看是否可以修改。如果可以，把config_generic结构类型的元素" max_connections "类型转换为config_int类型，修改variable成员为新值，修改该参数的来源source为当前来源PGC_S_FILE，如果元素" max_connections "的reset_source <= source，修改reset_val成员为新值，修改该参数的reset_source为当前来源PGC_S_FILE。
typedef enum
{
PGC_INTERNAL,
PGC_POSTMASTER,
PGC_SIGHUP,
PGC_BACKEND,
PGC_SUSET,
PGC_USERSET
} GucContext;
typedef enum
{
PGC_S_DEFAULT, /*wired-in default */
PGC_S_ENV_VAR, /*postmaster environment variable */
PGC_S_FILE, /*postgresql.conf */
PGC_S_ARGV, /*postmaster command line */
PGC_S_DATABASE, /*per-database setting */
PGC_S_USER, /*per-user setting */
PGC_S_CLIENT, /* fromclient connection request */
PGC_S_OVERRIDE, /*special case to forcibly set default */
PGC_S_INTERACTIVE, /* dividingline for error reporting */
PGC_S_TEST, /*test per-database or per-user setting */
PGC_S_SESSION /* SETcommand */
} GucSource
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