Avoid extra projection when scanning a compressed chunk

We don't have to project the result of the underlying scan of
DecompressChunk, because the latter can selectively decompress the
resulting columns anyway. The projection slows down the things
especially when we use JIT, when the projection has to be compiled for
each chunk.

To avoid this unnecessary work, use the physical targetlist for
compressed scan if we can. This commit rewrites the function that
determines what we have to decompress, and makes it use the targetlist
generated by the planner, instead of building it from scratch. Then we
can give it the physical targetlist when possible.

tsl/src/nodes/decompress_chunk/decompress_chunk.c

@@ -491,12 +491,24 @@ ts_decompress_chunk_generate_paths(PlannerInfo *root, RelOptInfo *chunk_rel, Hyp
 	compressed_rel->reloptkind = RELOPT_DEADREL;
 }
 
+/*
+ * Add a var for a particular column to the reltarget. attrs_used is a bitmap
+ * of which columns we already have in reltarget. We do not add the columns that
+ * are already there, and update it after adding something.
+ */
 static void
 compressed_reltarget_add_var_for_column(RelOptInfo *compressed_rel, Oid compressed_relid,
 										const char *column_name, Bitmapset **attrs_used)
 {
 	AttrNumber attnum = get_attnum(compressed_relid, column_name);
 	Assert(attnum > 0);
+
+	if (bms_is_member(attnum, *attrs_used))
+	{
+		/* This column is already in reltarget, we don't need duplicates. */
+		return;
+	}
+
 	*attrs_used = bms_add_member(*attrs_used, attnum);
 
 	Oid typid, collid;
@@ -518,8 +530,30 @@ compressed_rel_setup_reltarget(RelOptInfo *compressed_rel, CompressionInfo *info
 	Bitmapset *attrs_used = NULL;
 
 	Oid compressed_relid = info->compressed_rte->relid;
+
+	/*
+	 * We have to decompress three kinds of columns:
+	 * 1) output targetlist of the relation,
+	 * 2) columns required for the quals (WHERE),
+	 * 3) columns required for joins.
+	 */
+	List *exprs = list_copy(info->chunk_rel->reltarget->exprs);
 	ListCell *lc;
-	foreach (lc, info->chunk_rel->reltarget->exprs)
+	foreach (lc, info->chunk_rel->baserestrictinfo)
+	{
+		exprs = lappend(exprs, ((RestrictInfo *) lfirst(lc))->clause);
+	}
+	foreach (lc, info->chunk_rel->joininfo)
+	{
+		exprs = lappend(exprs, ((RestrictInfo *) lfirst(lc))->clause);
+	}
+
+	/*
+	 * Now go over the required expressions we prepared above, and add the
+	 * required columns to the compressed reltarget.
+	 */
+	info->compressed_rel->reltarget->exprs = NIL;
+	foreach (lc, exprs)
 	{
 		ListCell *lc2;
 		List *chunk_vars = pull_var_clause(lfirst(lc), PVC_RECURSE_PLACEHOLDERS);
@@ -531,9 +565,12 @@ compressed_rel_setup_reltarget(RelOptInfo *compressed_rel, CompressionInfo *info
 
 			/* skip vars that aren't from the uncompressed chunk */
 			if (chunk_var->varno != info->chunk_rel->relid)
+			{
 				continue;
+			}
 
-			/* if there's a system column or whole-row reference, add a whole-
+			/*
+			 * If there's a system column or whole-row reference, add a whole-
 			 * row reference, and we're done.
 			 */
 			if (chunk_var->varattno <= 0)

tsl/src/nodes/decompress_chunk/exec.c

@@ -144,7 +144,11 @@ initialize_column_state(DecompressChunkState *state)
 		next_compressed_scan_attno++;
 
 		AttrNumber output_attno = lfirst_int(lc);
-		Assert(output_attno != 0);
+		if (output_attno == 0)
+		{
+			/* We are asked not to decompress this column, skip it. */
+			continue;
+		}
 
 		DecompressChunkColumnState *column = &state->columns[state->num_columns];
 		state->num_columns++;