morningman commented on code in PR #69488:
URL: https://github.com/apache/doris/pull/69488#discussion_r4430911846
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fe/fe-core/src/main/java/org/apache/doris/nereids/trees/plans/commands/RowLevelDmlCommand.java:
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@@ -154,7 +154,7 @@ public void run(ConnectContext ctx, StmtExecutor
stmtExecutor) throws Exception
public Plan getExplainPlan(ConnectContext ctx) {
Review Comment:
Keeping this as is. A MERGE resolves `DEFAULT(column)` in its ON condition
against the writer schema it pins, whatever its WHEN branches, and so does
branch-4.2, which this PR ports. There, `IcebergMergeCommand` installs the
write schema context for every MERGE, delete-only ones included, both when it
runs and for EXPLAIN (`ExplainCommand` installs it again from the merge sink),
and `RewriteDefaultExpression` resolves a target column from it. So on
branch-4.2 too, `MERGE ... ON t.id = s.id AND t.c = DEFAULT(t.c) WHEN MATCHED
THEN DELETE` deletes the matched rows whose `c` is 35. Master evaluates
`DEFAULT(t.c)` to NULL there and deletes nothing.
To be precise about the DELETE thread
(https://github.com/apache/doris/pull/69488#discussion_r4409014234): the line
follows branch-4.2 and depends on the kind of statement. branch-4.2 gives a
DELETE no write schema context, so `DEFAULT(c)` fails there, and gives every
MERGE one. "Writes no row values" explains why the DELETE has none. It is not a
test applied to each MERGE. Rejecting a delete-only MERGE would break, after an
upgrade, a statement that runs on branch-4.1 and branch-4.2. It would also make
the same ON condition fail or resolve depending on whether the MERGE has an
UPDATE or INSERT branch, although the condition selects the same rows to delete
either way. The description now states that a MERGE resolves it whatever its
WHEN branches, also when they all delete.
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