Refactor index cost estimation functions in view of IndexClause changes.

Get rid of deconstruct_indexquals() in favor of just iterating over the
IndexClause list directly.  The extra services that that function used to
provide, such as hiding clause commutation and associating the right index
column with each clause, are no longer useful given the new data structure.
I'd originally thought that it'd provide a useful amount of abstraction
by freeing callers from paying attention to the exact clause type of each
indexqual, but that hope proves to have been vain, because few callers can
ignore the semantic differences between different clause types.  Indeed,
removing it results in a net code savings, and probably some cycles shaved
by not having to build an extra list-of-structs data structure.

Also, export a few formerly-static support functions, with the goal
of allowing extension AMs to write functionality equivalent to
genericcostestimate() without pointless code duplication.

Discussion: https://postgr.es/m/24586.1550106354@sss.pgh.pa.us

contrib/bloom/blcost.c

@@ -27,19 +27,15 @@ blcostestimate(PlannerInfo *root, IndexPath *path, double loop_count,
 			   double *indexPages)
 {
 	IndexOptInfo *index = path->indexinfo;
-	List	   *qinfos;
 	GenericCosts costs;
 
-	/* Do preliminary analysis of indexquals */
-	qinfos = deconstruct_indexquals(path);
-
 	MemSet(&costs, 0, sizeof(costs));
 
 	/* We have to visit all index tuples anyway */
 	costs.numIndexTuples = index->tuples;
 
 	/* Use generic estimate */
-	genericcostestimate(root, path, loop_count, qinfos, &costs);
+	genericcostestimate(root, path, loop_count, &costs);
 
 	*indexStartupCost = costs.indexStartupCost;
 	*indexTotalCost = costs.indexTotalCost;

src/include/utils/selfuncs.h

@@ -84,19 +84,6 @@ typedef struct VariableStatData
 	} while(0)
 
 
-/*
- * deconstruct_indexquals is a simple function to examine the indexquals
- * attached to a proposed IndexPath.  It returns a list of IndexQualInfo
- * structs, one per qual expression.
- */
-typedef struct
-{
-	RestrictInfo *rinfo;		/* the indexqual itself */
-	int			indexcol;		/* zero-based index column number */
-	Oid			clause_op;		/* qual's operator OID, if relevant */
-	Node	   *other_operand;	/* non-index operand of qual's operator */
-} IndexQualInfo;
-
 /*
  * genericcostestimate is a general-purpose estimator that can be used for
  * most index types.  In some cases we use genericcostestimate as the base
@@ -200,10 +187,13 @@ extern void estimate_hash_bucket_stats(PlannerInfo *root,
 						   Selectivity *mcv_freq,
 						   Selectivity *bucketsize_frac);
 
-extern List *deconstruct_indexquals(IndexPath *path);
+extern List *get_quals_from_indexclauses(List *indexclauses);
+extern Cost index_other_operands_eval_cost(PlannerInfo *root,
+							   List *indexquals);
+extern List *add_predicate_to_index_quals(IndexOptInfo *index,
+							 List *indexQuals);
 extern void genericcostestimate(PlannerInfo *root, IndexPath *path,
 					double loop_count,
-					List *qinfos,
 					GenericCosts *costs);
 
 /* Functions in array_selfuncs.c */