diff options
| -rw-r--r-- | src/backend/optimizer/plan/createplan.c | 18 |
1 files changed, 16 insertions, 2 deletions
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c index b3ca7a41b3..e49a07356d 100644 --- a/src/backend/optimizer/plan/createplan.c +++ b/src/backend/optimizer/plan/createplan.c @@ -6559,8 +6559,22 @@ make_limit(Plan *lefttree, Node *limitOffset, Node *limitCount, node->limitCount = limitCount; #ifdef XCP - if ((limitOffset == NULL || offset_est > 0) && - (limitCount == NULL || count_est > 0)) + /* + * We want to push down LIMTI clause to the remote side in order to limit + * the number of rows that get shipped from the remote side. This can be + * done even if there is an ORDER BY clause, as long as we fetch minimum + * number of rows from all the nodes and then do a local sort and apply the + * final limit. + * + * We can't push down limit clause unless its a constant. Similarly, if the + * OFFSET is specified then it must be a constant too. + * + * Simple expressions get folded into constants by the time we come here. + * So this works well in case of constant expressions such as + * SELECT .. LIMIT (1024 * 1024); + */ + if (limitCount && IsA(limitCount, Const) && + ((limitOffset == NULL) || IsA(limitOffset, Const))) { /* * We may reduce amount of rows sent over the network and do not send more |