Postgres：如何填充默认值？
Postgres: how does a default value get populated?
明确地说，我想知道用于填充默认值的机制是什么，而不是在 table 上创建默认值约束所需的 SQL 语法。
Postgres 是否使用某种触发器来更新默认值（如果缺少或其他原因）？我找不到关于 official website.
的解释
这发生在 src/backend/rewrite/rewriteHandler.c 中的 rewriteTargetListIU 函数中。评论说明了一切：
/*
* rewriteTargetListIU - rewrite INSERT/UPDATE targetlist into standard form
*
* This has the following responsibilities:
*
* 1. For an INSERT, add tlist entries to compute default values for any
* attributes that have defaults and are not assigned to in the given tlist.
* (We do not insert anything for default-less attributes, however. The
* planner will later insert NULLs for them, but there's no reason to slow
* down rewriter processing with extra tlist nodes.) Also, for both INSERT
* and UPDATE, replace explicit DEFAULT specifications with column default
* expressions.
所以这发生在 查询重写 期间，这是解析 SQL 字符串和优化它之间的步骤。
明确地说，我想知道用于填充默认值的机制是什么，而不是在 table 上创建默认值约束所需的 SQL 语法。
Postgres 是否使用某种触发器来更新默认值（如果缺少或其他原因）？我找不到关于 official website.
的解释这发生在 src/backend/rewrite/rewriteHandler.c 中的 rewriteTargetListIU 函数中。评论说明了一切：
/*
* rewriteTargetListIU - rewrite INSERT/UPDATE targetlist into standard form
*
* This has the following responsibilities:
*
* 1. For an INSERT, add tlist entries to compute default values for any
* attributes that have defaults and are not assigned to in the given tlist.
* (We do not insert anything for default-less attributes, however. The
* planner will later insert NULLs for them, but there's no reason to slow
* down rewriter processing with extra tlist nodes.) Also, for both INSERT
* and UPDATE, replace explicit DEFAULT specifications with column default
* expressions.
所以这发生在 查询重写 期间，这是解析 SQL 字符串和优化它之间的步骤。