Postgresql 中文操作指南
SPI_cursor_open
SPI_cursor_open——使用通过 SPI_prepare 创建的语句设置一个游标
SPI_cursor_open — set up a cursor using a statement created with SPI_prepare
Synopsis
Portal SPI_cursor_open(const char * name, SPIPlanPtr plan,
Datum * values, const char * nulls,
bool read_only)Description
SPI_cursor_open 设置一个游标（在内部，一个门户）用于执行 SPI_prepare 准备的语句。这些参数与 SPI_execute_plan 的对应参数意义相同。
SPI_cursor_open sets up a cursor (internally, a portal) that will execute a statement prepared by SPI_prepare. The parameters have the same meanings as the corresponding parameters to SPI_execute_plan.
使用游标而不是直接执行语句有两个好处。首先，可以一次检索少量结果行，避免为返回多数行的查询导致内存溢出。其次，一个门户在当前 C 函数完成后仍然有效（实际上，它可以在当前事务结束时仍然有效）。将门户名称返回到 C 函数的调用者提供了一种以结果的方式返回一组行。
Using a cursor instead of executing the statement directly has two benefits. First, the result rows can be retrieved a few at a time, avoiding memory overrun for queries that return many rows. Second, a portal can outlive the current C function (it can, in fact, live to the end of the current transaction). Returning the portal name to the C function’s caller provides a way of returning a row set as result.
传入的参数数据将被复制到光标的门户中，因此可在光标仍然存在时释放它。
The passed-in parameter data will be copied into the cursor’s portal, so it can be freed while the cursor still exists.
Arguments
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const char * _name_
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name for portal, or NULL to let the system select a name
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SPIPlanPtr _plan_
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prepared statement (returned by SPI_prepare)
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Datum * _values_
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An array of actual parameter values. Must have same length as the statement’s number of arguments.
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const char * _nulls_
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An array describing which parameters are null. Must have same length as the statement’s number of arguments.
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If nulls is NULL then SPI_cursor_open assumes that no parameters are null. Otherwise, each entry of the nulls array should be ' ' if the corresponding parameter value is non-null, or 'n' if the corresponding parameter value is null. (In the latter case, the actual value in the corresponding values entry doesn’t matter.) Note that nulls is not a text string, just an array: it does not need a '\0' terminator.
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bool _read_only_
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true for read-only execution
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