Avoid using SPI in RI trigger functions

Currently, ri_PlanCheck() uses SPI_prepare() to create an SPI plan
containing a CachedPlanSource for the SQL query used in RI checks.
Similarly, ri_PerformCheck() calls SPI_execute_snapshot() to
execute the query with a specific snapshot.

This commit introduces ri_PlanCreate() and ri_PlanExecute() to
replace SPI_prepare() and SPI_execute_snapshot(), respectively.

* ri_PlanCreate()

Creates an "RI plan" for a query using a caller-specified callback
function, such as ri_SqlStringPlanCreate(), which produces a
CachedPlanSource for the input SQL string. This mirrors SPI_prepare()
functionality.

* ri_PlanExecute()

Executes an "RI plan" using a callback saved within the "RI plan"
(struct RIPlan). For example, ri_SqlStringPlanExecute() fetches a
CachedPlan from the CachedPlanSource and executes its PlannedStmt
using the executor. Snapshot handling is now fully managed by
ri_PerformCheck(), eliminating dependence on SPI's snapshot logic.

These changes make ri_PlanCreate() and ri_PlanExecute() pluggable,
laying the groundwork for future commits to replace SQL-based RI
checks with optimized C functions for direct table/index scans.

Note:

This only addresses RI_* functions addressed from RI triggers and
not those called from tablecmds.c, such as RI_Initial_Check() and
RI_PartitionRemove_Check(), which still use SPI_prepare() and
SPI_execute_snapshot().

Reviewed-by: Robert Haas
Discussion: https://postgr.es/m/CA+HiwqGkfJfYdeq5vHPh6eqPKjSbfpDDY+j-kXYFePQedtSLeg@mail.gmail.com
Discussion: https://postgr.es/m/CA+HiwqG5e8pk8s7+7zhr1Nc_PGyhEdM5f=pHkMOdK1RYWXfJsg@mail.gmail.com

Author: amitlan

src/backend/executor/spi.c

@@ -765,7 +765,7 @@ SPI_execute_plan_with_paramlist(SPIPlanPtr plan, ParamListInfo params,
  * end of the command.
  *
  * This is currently not documented in spi.sgml because it is only intended
- * for use by RI triggers.
+ * for use by some functions in ri_triggers.c.
  *
  * Passing snapshot == InvalidSnapshot will select the normal behavior of
  * fetching a new snapshot for each query.