Shulou(Shulou.com)06/02 Report--

In this issue, the editor will bring you about the initial definition and initialization of PostgreSQL. The article is rich in content and analyzes and narrates it from a professional point of view. I hope you can get something after reading this article.

Configuration parameters (or GUC variables, grand unified configuration) are common settings that appear in postgresql.conf. For more information, please see document 19. Server Configuration. I focus on how they are set and play a role in the code.

It mainly talks about different types of initial definitions.

1. Corresponding structures of different types

In addition to the general structure mentioned in the previous article, each type has its own definition, such as Boolean:

Struct config_bool {struct config_generic gen; / * constant fields, must be set correctly in initial value: * / bool * variable; bool boot_val; GucBoolCheckHook check_hook; GucBoolAssignHook assign_hook; GucShowHook show_hook; / * variable fields, initialized at runtime: * / bool reset_val; void * reset_extra;}

The first member of all type structures is a config_generic structure named gen, such as an integer:

Struct config_int {struct config_generic gen; / * constant fields, must be set correctly in initial value: * /..

Such code designs can be seen everywhere in PG and can be understood as inheritance similar to C++. In the process of use, you can know the actual type of a structure pointer according to the value of the gen member, such as:

Struct config_generic * conf;... Conf = guc_ variables [varnum]; / * now get the type specific attributes * / switch (conf- > vartype) {case PGC_BOOL: {struct config_bool * lconf = (struct config_bool *) conf ... / * boot_val * / values [12] = pstrdup (lconf- > boot_val? On: "off");

When you read that a config_generic structure member vartype is PGC_BOOL, you can know that it actually points to a config_bool type structure.

2. Initial definition of configuration parameters (src/backend/utils/misc/guc.c)

The Boolean configuration parameter is defined as the structure array ConfigureNamesBool:

Static struct config_bool ConfigureNamesBool [] = {{"enable_seqscan", PGC_USERSET, QUERY_TUNING_METHOD, gettext_noop ("Enables the planner's use of sequential-scan plans."), NULL}, & enable_seqscan, true, NULL, NULL NULL},...

The first part is the definition of gen member, which is mentioned in the previous article: configuration parameter name, context, classification, short description, long description, their values are: enable_seqscan, PGC_USERSET (allows ordinary users to set), QUERY_TUNING_METHOD (Query Tuning / Planner Method Configuration), and so on.

Other types also have their own array of definitions:

Static struct config_int ConfigureNamesInt [] = static struct config_real ConfigureNamesReal [] =

Wait, I won't repeat it.

Each parameter has a member boot_val, the initial value at startup, and its type is the same as the parameter, such as bool in config_bool and int in config_int. In the above example, the initial value of enable_seqscan is true, which is consistent with the default value in postgresql.conf.

3. Type initialization at startup

Function build_guc_variables ()

For (I = 0; ConfigureNamesInt [I] .gen.name; ifigureNamesInt [I]) {struct config_int * conf = & ConfigureNamesInt [I]; conf- > gen.vartype = PGC_INT; num_vars++;}

Parameter types are given here, and each array holds different types of parameters, so you don't need to specify them one by one at the time of definition, just define the values of the first few members.

All different types of parameter pointers are placed in the same array and sorted by name.

4. Initialization of the value at startup

The type initialization function is called in InitializeGUCOptions (), and InitializeOneGUCOption is immediately called for each parameter.

Take string parameters as an example

If (conf- > boot_val! = NULL) newval = guc_strdup (FATAL, conf- > boot_val); else newval = NULL If (! call_string_check_hook (conf, & newval, & extra, PGC_S_DEFAULT, LOG) elog (FATAL, "failed to initialize% s to\"% s\ "" Conf- > gen.name, newval? Newval: ""); if (conf- > assign_hook) conf- > assign_hook (newval, extra); * conf- > variable = conf- > reset_val = newval; conf- > gen.extra = conf- > reset_extra = extra

It's simple here to initialize the other members with boot_val.

You must have noticed that there are three definitions of hook in the structure, and two are used here. If you are interested, you can find out for yourself.

5. The influence of environment variables on configuration

See the function InitializeGUCOptionsFromEnvironment, which is no longer expanded here, but will be discussed together with the SET command or the read of the configuration file.

The above is the initialization of the configuration parameters when the database process starts (including the startup of the back-end process). It is written very thick and may be omitted.

This is the initial definition and initialization of PostgreSQL shared by the editor. If you happen to have similar doubts, you might as well refer to the above analysis to understand. If you want to know more about it, you are welcome to follow the industry information channel.

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