Faster expression evaluation and targetlist projection.

This replaces the old, recursive tree-walk based evaluation, with
non-recursive, opcode dispatch based, expression evaluation.
Projection is now implemented as part of expression evaluation.

This both leads to significant performance improvements, and makes
future just-in-time compilation of expressions easier.

The speed gains primarily come from:
- non-recursive implementation reduces stack usage / overhead
- simple sub-expressions are implemented with a single jump, without
  function calls
- sharing some state between different sub-expressions
- reduced amount of indirect/hard to predict memory accesses by laying
  out operation metadata sequentially; including the avoidance of
  nearly all of the previously used linked lists
- more code has been moved to expression initialization, avoiding
  constant re-checks at evaluation time

Future just-in-time compilation (JIT) has become easier, as
demonstrated by released patches intended to be merged in a later
release, for primarily two reasons: Firstly, due to a stricter split
between expression initialization and evaluation, less code has to be
handled by the JIT. Secondly, due to the non-recursive nature of the
generated "instructions", less performance-critical code-paths can
easily be shared between interpreted and compiled evaluation.

The new framework allows for significant future optimizations. E.g.:
- basic infrastructure for to later reduce the per executor-startup
  overhead of expression evaluation, by caching state in prepared
  statements.  That'd be helpful in OLTPish scenarios where
  initialization overhead is measurable.
- optimizing the generated "code". A number of proposals for potential
  work has already been made.
- optimizing the interpreter. Similarly a number of proposals have
  been made here too.

The move of logic into the expression initialization step leads to some
backward-incompatible changes:
- Function permission checks are now done during expression
  initialization, whereas previously they were done during
  execution. In edge cases this can lead to errors being raised that
  previously wouldn't have been, e.g. a NULL array being coerced to a
  different array type previously didn't perform checks.
- The set of domain constraints to be checked, is now evaluated once
  during expression initialization, previously it was re-built
  every time a domain check was evaluated. For normal queries this
  doesn't change much, but e.g. for plpgsql functions, which caches
  ExprStates, the old set could stick around longer.  The behavior
  around might still change.

Author: Andres Freund, with significant changes by Tom Lane,
	changes by Heikki Linnakangas
Reviewed-By: Tom Lane, Heikki Linnakangas
Discussion: https://postgr.es/m/20161206034955.bh33paeralxbtluv@alap3.anarazel.de

src/backend/utils/cache/typcache.c

@@ -96,11 +96,11 @@ static TypeCacheEntry *firstDomainTypeEntry = NULL;
  * this struct for the common case of a constraint-less domain; we just set
  * domainData to NULL to indicate that.
  *
- * Within a DomainConstraintCache, we abuse the DomainConstraintState node
- * type a bit: check_expr fields point to expression plan trees, not plan
- * state trees.  When needed, expression state trees are built by flat-copying
- * the DomainConstraintState nodes and applying ExecInitExpr to check_expr.
- * Such a state tree is not part of the DomainConstraintCache, but is
+ * Within a DomainConstraintCache, we store expression plan trees, but the
+ * check_exprstate fields of the DomainConstraintState nodes are just NULL.
+ * When needed, expression evaluation nodes are built by flat-copying the
+ * DomainConstraintState nodes and applying ExecInitExpr to check_expr.
+ * Such a node tree is not part of the DomainConstraintCache, but is
  * considered to belong to a DomainConstraintRef.
  */
 struct DomainConstraintCache
@@ -779,8 +779,8 @@ load_domaintype_info(TypeCacheEntry *typentry)
 			r = makeNode(DomainConstraintState);
 			r->constrainttype = DOM_CONSTRAINT_CHECK;
 			r->name = pstrdup(NameStr(c->conname));
-			/* Must cast here because we're not storing an expr state node */
-			r->check_expr = (ExprState *) check_expr;
+			r->check_expr = check_expr;
+			r->check_exprstate = NULL;
 
 			MemoryContextSwitchTo(oldcxt);
 
@@ -859,6 +859,7 @@ load_domaintype_info(TypeCacheEntry *typentry)
 		r->constrainttype = DOM_CONSTRAINT_NOTNULL;
 		r->name = pstrdup("NOT NULL");
 		r->check_expr = NULL;
+		r->check_exprstate = NULL;
 
 		/* lcons to apply the nullness check FIRST */
 		dcc->constraints = lcons(r, dcc->constraints);
@@ -946,8 +947,8 @@ prep_domain_constraints(List *constraints, MemoryContext execctx)
 		newr = makeNode(DomainConstraintState);
 		newr->constrainttype = r->constrainttype;
 		newr->name = r->name;
-		/* Must cast here because cache items contain expr plan trees */
-		newr->check_expr = ExecInitExpr((Expr *) r->check_expr, NULL);
+		newr->check_expr = r->check_expr;
+		newr->check_exprstate = ExecInitExpr(r->check_expr, NULL);
 
 		result = lappend(result, newr);
 	}
@@ -962,13 +963,18 @@ prep_domain_constraints(List *constraints, MemoryContext execctx)
  *
  * Caller must tell us the MemoryContext in which the DomainConstraintRef
  * lives.  The ref will be cleaned up when that context is reset/deleted.
+ *
+ * Caller must also tell us whether it wants check_exprstate fields to be
+ * computed in the DomainConstraintState nodes attached to this ref.
+ * If it doesn't, we need not make a copy of the DomainConstraintState list.
  */
 void
 InitDomainConstraintRef(Oid type_id, DomainConstraintRef *ref,
-						MemoryContext refctx)
+						MemoryContext refctx, bool need_exprstate)
 {
 	/* Look up the typcache entry --- we assume it survives indefinitely */
 	ref->tcache = lookup_type_cache(type_id, TYPECACHE_DOMAIN_INFO);
+	ref->need_exprstate = need_exprstate;
 	/* For safety, establish the callback before acquiring a refcount */
 	ref->refctx = refctx;
 	ref->dcc = NULL;
@@ -980,8 +986,11 @@ InitDomainConstraintRef(Oid type_id, DomainConstraintRef *ref,
 	{
 		ref->dcc = ref->tcache->domainData;
 		ref->dcc->dccRefCount++;
-		ref->constraints = prep_domain_constraints(ref->dcc->constraints,
-												   ref->refctx);
+		if (ref->need_exprstate)
+			ref->constraints = prep_domain_constraints(ref->dcc->constraints,
+													   ref->refctx);
+		else
+			ref->constraints = ref->dcc->constraints;
 	}
 	else
 		ref->constraints = NIL;
@@ -1032,8 +1041,11 @@ UpdateDomainConstraintRef(DomainConstraintRef *ref)
 		{
 			ref->dcc = dcc;
 			dcc->dccRefCount++;
-			ref->constraints = prep_domain_constraints(dcc->constraints,
-													   ref->refctx);
+			if (ref->need_exprstate)
+				ref->constraints = prep_domain_constraints(dcc->constraints,
+														   ref->refctx);
+			else
+				ref->constraints = dcc->constraints;
 		}
 	}
 }