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robertmhaasrobertmhaas
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postgres_fdw: Clean up handling of system columns.
Previously, querying the xmin column of a single postgres_fdw foreign table fetched the tuple length, xmax the typmod, and cmin or cmax the composite type OID of the tuple. However, when you queried several such tables and the join got shipped to the remote side, these columns ended up containing the remote values of the corresponding columns. Both behaviors are rather unprincipled, the former for obvious reasons and the latter because the remote values of these columns don't have any local significance; our transaction IDs are in a different space than those of the remote machine. Clean this up by setting all of these fields to 0 in both cases. Also fix the handling of tableoid to be sane. Robert Haas and Ashutosh Bapat, reviewed by Etsuro Fujita.
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‎contrib/postgres_fdw/deparse.c

Lines changed: 45 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1571,13 +1571,38 @@ deparseColumnRef(StringInfo buf, int varno, int varattno, PlannerInfo *root,
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{
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RangeTblEntry *rte;
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1574-
/* varattno can be a whole-row reference, ctid or a regular table column */
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if (varattno == SelfItemPointerAttributeNumber)
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{
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/* We support fetching the remote side's CTID. */
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if (qualify_col)
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ADD_REL_QUALIFIER(buf, varno);
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appendStringInfoString(buf, "ctid");
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}
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else if (varattno < 0)
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{
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/*
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* All other system attributes are fetched as 0, except for table OID,
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* which is fetched as the local table OID. However, we must be
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* careful; the table could be beneath an outer join, in which case
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* it must go to NULL whenever the rest of the row does.
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*/
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Oid fetchval = 0;
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if (varattno == TableOidAttributeNumber)
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{
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rte = planner_rt_fetch(varno, root);
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fetchval = rte->relid;
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}
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if (qualify_col)
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{
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appendStringInfoString(buf, "CASE WHEN ");
1600+
ADD_REL_QUALIFIER(buf, varno);
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appendStringInfo(buf, "* IS NOT NULL THEN %u END", fetchval);
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}
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else
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appendStringInfo(buf, "%u", fetchval);
1605+
}
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else if (varattno == 0)
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{
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/* Whole row reference */
@@ -1606,10 +1631,29 @@ deparseColumnRef(StringInfo buf, int varno, int varattno, PlannerInfo *root,
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*/
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attrs_used = bms_add_member(NULL,
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0 - FirstLowInvalidHeapAttributeNumber);
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/*
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* In case the whole-row reference is under an outer join then it has to
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* go NULL whenver the rest of the row goes NULL. Deparsing a join query
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* would always involve multiple relations, thus qualify_col would be
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* true.
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*/
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if (qualify_col)
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{
1643+
appendStringInfoString(buf, "CASE WHEN ");
1644+
ADD_REL_QUALIFIER(buf, varno);
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appendStringInfo(buf, "* IS NOT NULL THEN ");
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}
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appendStringInfoString(buf, "ROW(");
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deparseTargetList(buf, root, varno, rel, false, attrs_used, qualify_col,
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&retrieved_attrs);
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appendStringInfoString(buf, ")");
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/* Complete the CASE WHEN statement started above. */
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if (qualify_col)
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appendStringInfo(buf," END");
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heap_close(rel, NoLock);
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bms_free(attrs_used);
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}

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