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A check that passes on blank counters is worse than no check (#753) #823

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jdatcmd merged 2 commits into from
Aug 28, 2026
Merged

A check that passes on blank counters is worse than no check (#753) #823

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test: refuse blank counters, and order the scan premise before them (…
jdatcmd Aug 28, 2026
3a1f8e8
test: let the group-counter check defend itself, not lean on a premise
jdatcmd Aug 28, 2026
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27 changes: 22 additions & 5 deletions test/doc_parallel_premise.sh
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Original file line number Diff line number Diff line change
Expand Up @@ -114,16 +114,35 @@ serial_cost() {
| grep -oE 'cost=[0-9.]+\.\.[0-9.]+' | sed 's/.*\.\.//' | head -1
}

# The scan-node premise comes FIRST, because every counter below is read out of
# a columnar scan's EXPLAIN output and does not exist without one.
for _c in "$doc_col" sel; do
check "premise: the $_c plan is exactly one columnar scan node" \
"$(scan_lines "$_c")" "1"
done

doc_read=$(read_groups "$doc_col"); doc_tot=$(total_groups "$doc_col")
sel_read=$(read_groups sel); sel_tot=$(total_groups sel)

# A counter that was not printed is an EMPTY string, and `check "" ""` PASSES.
# That is not hypothetical: with pgcolumnar.enable_custom_scan=off the plan is a
# seq scan, every counter below is blank, and check 1 passed comparing one blank
# with another. A check that passes on nothing is worse than no check.
check "premise: the group counters were read, not blank" \
"$([ -n "$doc_read" ] && [ -n "$doc_tot" ] && [ -n "$sel_read" ] && [ -n "$sel_tot" ] \
&& echo yes || echo "doc=$doc_read/$doc_tot sel=$sel_read/$sel_tot")" "yes"
echo "-- $doc_col (the documented column): $doc_read of $doc_tot groups read"
echo "-- sel (stored in order): $sel_read of $sel_tot groups read"

# --- 1. the defect this suite exists for ------------------------------------
# The published speedup numbers were taken with every group read. If the page's
# query prunes, the numbers beside it describe a different amount of work.
# Defaults that CANNOT collide, so blank-versus-blank fails on its own rather
# than relying on the premise above to catch it. `check "" ""` passes, so a check
# fed by two extracted values must never be able to compare one blank with
# another.
check "the documented narrow query reads every chunk group" \
"$doc_read" "$doc_tot"
"${doc_read:-<no-read-counter>}" "${doc_tot:-<no-total-counter>}"

# --- 2. the contrast the page now documents is real -------------------------
check "and filtering the ordered column instead prunes some away" \
Expand All @@ -141,11 +160,9 @@ check "the documented column is not the one stored in order" \
# Exact rather than bounded: reading more groups must cost more. A bound here
# would need calibrating and the direction does not.
if [ "$doc_col" != "sel" ]; then
for _c in "$doc_col" sel; do
check "premise: the $_c plan is exactly one columnar scan node" \
"$(scan_lines "$_c")" "1"
done
dc=$(serial_cost "$doc_col"); sc=$(serial_cost sel)
check "premise: both costs were extracted, not blank" \
"$([ -n "$dc" ] && [ -n "$sc" ] && echo yes || echo "dc=$dc sc=$sc")" "yes"
echo "-- serial cost: $doc_col $dc, sel $sc"
check "and the documented column is costed above the ordered one" \
"$(awk -v a="$dc" -v b="$sc" 'BEGIN { print (a > b) ? "yes" : "no (" a " vs " b ")" }')" "yes"
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