Db2 fdw percentile push down#120
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… result-type crash foreign_expr_walker now recurses into Aggref.aggdirectargs (previously unwalked) and consults a shared db2AggregateInfo() helper so it only approves aggregates the deparser actually knows how to emit. deparseAggref gains deparseOrderedSetAggref() to build DB2's FUNC(fraction) WITHIN GROUP (ORDER BY col ASC|DESC) syntax for percentile_cont/percentile_disc, restricted to the single-column, non-array form DB2 supports. While testing this against the live DB2 instance, found that getUsedColumns() derived every pushed-down aggregate's result column type by borrowing its argument column's DB2 type. That's only correct when the aggregate preserves its input type (MIN/MAX, PERCENTILE_DISC); for AVG/SUM/PERCENTILE_CONT the output type differs, which crammed DB2's DECFLOAT-formatted result into a wrongly-typed, undersized buffer - crashing the backend on AVG(bigint_col) and producing a parse error for PERCENTILE_CONT. Now re-tags the result column with the aggregate's real return type when it differs from the borrowed argument type, routing it through the existing safe SQLGetData/text-fetch path. Verified against live DB2: PERCENTILE_CONT/PERCENTILE_DISC push down correctly (index-only scan), AVG(bigint) no longer crashes, and existing MIN/MAX/AVG/SUM/COUNT pushdown output is unchanged. Added real PERCENTILE_CONT/PERCENTILE_DISC assertions to tc015.sql and refreshed expected output. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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