Postgresql 中文操作指南
Description
SPI_saveplan 将传递的语句（由 SPI_prepare 准备）复制到不会由 SPI_finish 或事务管理器释放的内存中，并返回已复制语句的指针。这样，您便可以在当前会话中再次调用 C 函数时重新使用已准备的语句。
SPI_saveplan copies a passed statement (prepared by SPI_prepare) into memory that will not be freed by SPI_finish nor by the transaction manager, and returns a pointer to the copied statement. This gives you the ability to reuse prepared statements in the subsequent invocations of your C function in the current session.
Return Value
复制的语句的指针；如果失败，则为 NULL 。出现错误时，将 SPI_result 设置如下：
Pointer to the copied statement; or NULL if unsuccessful. On error, SPI_result is set thus:
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SPI_ERROR_ARGUMENT
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if plan is NULL or invalid
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SPI_ERROR_UNCONNECTED
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if called from an unconnected C function
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Notes
最初传递的语句不会释放，所以您可能希望对其执行 SPI_freeplan ，以便避免在 SPI_finish 之前泄露内存。
The originally passed-in statement is not freed, so you might wish to do SPI_freeplan on it to avoid leaking memory until SPI_finish.
在大多数情况下， SPI_keepplan 优于此函数，因为它实现了基本相同的结果，而无需实际复制已准备语句的数据结构。
In most cases, SPI_keepplan is preferred to this function, since it accomplishes largely the same result without needing to physically copy the prepared statement’s data structures.