PostgreSQL启动过程中的那些事三：加载GUC参数
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,
