[Postgres-xc-developers] COPY and BR triggers

[Amit K. <ami...@en...>]

In XC, the way COPY is implemented is that for each record, we read the
whole line into memory, and then pass it to the datanode as-is. If there
are non-shippable default column expressions, we evaluate the default
values , convert them into output form, and append them to the data row.

In presence of BR triggers, currently the ExecBRInsertTriggers() do not get
called because of the way we skip the whole PG code block; instead we just
send the data row as-is, optionally appending default values into the data
row.

What we need to do is; convert the tuple returned by ExecBRTriggers into
text data row, but the text data should be in COPY format. This is because
we need to send the data row to the datanode using COPY command, so it
requires correct COPY format, such as escape sequences.

For this, we need to call the function CopyOneRowTo() that is being used by
COPY TO. This will make sure it will emit the data row in the COPY format.
But we need to create a temporary CopyState because CopyOneRowTo() needs
it. We can derive it from the current CopyState that is already created for
COPY FROM. Most of the fields remain the same, except we need to re-assign
CopyState->line_buf, and CopyState->rowcontext.

This will save us from writing code to make sure the new output data row
generated by BR triggers complies with COPY data format.

I had already done similar thing for appending default values into the data
row. We call functions like CopyAttributeOutCSV(), CopyInt32() to append
the values to the data row in COPY format. There, we did not require
CopyOneRow() because we did not require the complete row, we needed to
append only a subset of columns to the existing data row.

Comments/suggestions welcome.