Function ExecReadyInterpretedExpr

src/backend/executor/execExprInterp.c:234–382

* Prepare ExprState for interpreted execution. */

 * Prepare ExprState for interpreted execution.
 */
void
ExecReadyInterpretedExpr(ExprState *state)
{
	/* Ensure one-time interpreter setup has been done */
	ExecInitInterpreter();

	/* Simple validity checks on expression */
	Assert(state->steps_len >= 1);
	Assert(state->steps[state->steps_len - 1].opcode == EEOP_DONE);

	/*
	 * Don't perform redundant initialization. This is unreachable in current
	 * cases, but might be hit if there's additional expression evaluation
	 * methods that rely on interpreted execution to work.
	 */
	if (state->flags & EEO_FLAG_INTERPRETER_INITIALIZED)
		return;

	/*
	 * First time through, check whether attribute matches Var.  Might not be
	 * ok anymore, due to schema changes. We do that by setting up a callback
	 * that does checking on the first call, which then sets the evalfunc
	 * callback to the actual method of execution.
	 */
	state->evalfunc = ExecInterpExprStillValid;

	/* DIRECT_THREADED should not already be set */
	Assert((state->flags & EEO_FLAG_DIRECT_THREADED) == 0);

	/*
	 * There shouldn't be any errors before the expression is fully
	 * initialized, and even if so, it'd lead to the expression being
	 * abandoned.  So we can set the flag now and save some code.
	 */
	state->flags |= EEO_FLAG_INTERPRETER_INITIALIZED;

	/*
	 * Select fast-path evalfuncs for very simple expressions.  "Starting up"
	 * the full interpreter is a measurable overhead for these, and these
	 * patterns occur often enough to be worth optimizing.
	 */
	if (state->steps_len == 3)
	{
		ExprEvalOp	step0 = state->steps[0].opcode;
		ExprEvalOp	step1 = state->steps[1].opcode;

		if (step0 == EEOP_INNER_FETCHSOME &&
			step1 == EEOP_INNER_VAR)
		{
			state->evalfunc_private = (void *) ExecJustInnerVar;
			return;
		}
		else if (step0 == EEOP_OUTER_FETCHSOME &&
				 step1 == EEOP_OUTER_VAR)
		{
			state->evalfunc_private = (void *) ExecJustOuterVar;
			return;
		}