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avamingliavamingli
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Optimize MPP FDW LIMIT/OFFSET push down when there is NULL/0. (#17246)
When there are NULL or zero values in OFFSET/LIMIT clause, we do NOT need to fetch more rows and expression NULL plus others is pointless. Optimize (N > 0) LIMIT 0 OFFSET N to LIMIT 0 LIMIT 0 OFFSET NULL to LIMIT 0 LIMIT N OFFSET NULL to LIMIT N LIMIT N OFFSET 0 to LIMIT N Example before this fix: EXPLAIN (VERBOSE, COSTS OFF) SELECT c1, c2 FROM mpp_ft2 order by c1 limit 0 offset 998; QUERY PLAN -------------- Limit Output: c1, c2 -> Gather Motion 2:1 (slice1; segments: 2) Output: c1, c2 Merge Key: c1 -> Foreign Scan on public.mpp_ft2 Output: c1, c2 Remote SQL: SELECT c1, c2 FROM "MPP_S 1"."T 2" ORDER BY c1 ASC NULLS LAST LIMIT (998::bigint + 0::bigint) We will have to fetch 998 rows from remote, but as we have limit 0, that's pointless. With this fix: EXPLAIN (VERBOSE, COSTS OFF) SELECT c1, c2 FROM mpp_ft2 order by c1 limit 0 offset 998; QUERY PLAN -------------- Limit Output: c1, c2 -> Gather Motion 2:1 (slice1; segments: 2) Output: c1, c2 Merge Key: c1 -> Foreign Scan on public.mpp_ft2 Output: c1, c2 Remote SQL: SELECT c1, c2 FROM "MPP_S 1"."T 2" ORDER BY c1 ASC NULLS LAST LIMIT 0::bigint
1 parent cc09dbd commit eeea7f1

4 files changed

Lines changed: 136 additions & 3 deletions

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‎contrib/postgres_fdw/Makefile‎

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -27,7 +27,7 @@ endif
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# For postgres_fdw test
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export PG_PORT=5432
30-
installcheck: prep_postgres
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installcheck: install prep_postgres
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clean: clean_postgres
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prep_postgres:
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./postgres_setup.bash

‎contrib/postgres_fdw/deparse.c‎

Lines changed: 19 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -3544,14 +3544,31 @@ appendLimitClause(deparse_expr_cxt *context)
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* This may reduce the number of tuples that we need to fetch from remote servers.
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*/
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Node *precount = copyObject(root->parse->limitCount);
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Node *offset = copyObject(root->parse->limitOffset);
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bool offset_needed = ((precount != NULL) && (offset != NULL));
35473549

35483550
/*
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* If we've specified both OFFSET and LIMIT clause,
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* it's enough to fetch tuples from 0 to limitCount + limitOffset from remote servers.
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* optimize (N > 0):
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* LIMIT 0 OFFSET N to LIMIT 0
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* LIMIT 0 OFFSET NULL to LIMIT 0
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* LIMIT N OFFSET NULL to LIMIT N
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* LIMIT N OFFSET 0 to LIMIT N
35513558
*/
3559+
if (offset_needed &&
3560+
IsA(precount, Const) &&
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(((Const *) precount)->constisnull || ((Const *) precount)->constvalue == 0))
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offset_needed = false;
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3564+
if (offset_needed &&
3565+
IsA(offset, Const) &&
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(((Const *) offset)->constisnull || ((Const *) offset)->constvalue == 0))
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offset_needed = false;
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35523569
if (precount)
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{
3554-
if (root->parse->limitOffset)
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if (offset_needed)
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{
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ParseState *pstate = make_parsestate(NULL);
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/*
@@ -3560,7 +3577,7 @@ appendLimitClause(deparse_expr_cxt *context)
35603577
*/
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precount = (Node *) make_op(pstate,
35623579
list_make2(makeString("pg_catalog"), makeString(pstrdup("+"))),
3563-
copyObject(root->parse->limitOffset),
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offset,
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precount,
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NULL,
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-1);

‎contrib/postgres_fdw/expected/mpp_gp2pg_postgres_fdw.out‎

Lines changed: 99 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -981,6 +981,105 @@ SELECT c1, c2 FROM mpp_ft2 order by c1 offset 998;
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1000 | 0
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(2 rows)
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-- test LIMIT 0, OFFSET null/0
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ALTER FOREIGN TABLE mpp_ft2 OPTIONS(set use_remote_estimate 'true');
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EXPLAIN (VERBOSE, COSTS OFF)
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SELECT c1, c2 FROM mpp_ft2 order by c1 limit null offset 998;
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QUERY PLAN
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------------------------------------------------------------------------------------------------------------
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Limit
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Output: c1, c2
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-> Gather Motion 2:1 (slice1; segments: 2)
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Output: c1, c2
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Merge Key: c1
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-> Foreign Scan on public.mpp_ft2
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Output: c1, c2
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Remote SQL: SELECT c1, c2 FROM "MPP_S 1"."T 2" ORDER BY c1 ASC NULLS LAST LIMIT NULL::bigint
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Optimizer: Postgres-based planner
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Settings: gp_enable_minmax_optimization = 'off'
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(10 rows)
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EXPLAIN (VERBOSE, COSTS OFF)
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SELECT c1, c2 FROM mpp_ft2 order by c1 limit all offset 998;
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QUERY PLAN
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------------------------------------------------------------------------------------------------------------
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Limit
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Output: c1, c2
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-> Gather Motion 2:1 (slice1; segments: 2)
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Output: c1, c2
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Merge Key: c1
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-> Foreign Scan on public.mpp_ft2
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Output: c1, c2
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Remote SQL: SELECT c1, c2 FROM "MPP_S 1"."T 2" ORDER BY c1 ASC NULLS LAST LIMIT NULL::bigint
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Optimizer: Postgres-based planner
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Settings: gp_enable_minmax_optimization = 'off'
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(10 rows)
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EXPLAIN (VERBOSE, COSTS OFF)
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SELECT c1, c2 FROM mpp_ft2 order by c1 limit 0 offset 998;
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QUERY PLAN
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-------------------------------------------------------------------------------------------------------------------------
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Limit
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Output: c1, c2
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-> Gather Motion 2:1 (slice1; segments: 2)
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Output: c1, c2
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Merge Key: c1
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-> Foreign Scan on public.mpp_ft2
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Output: c1, c2
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Remote SQL: SELECT c1, c2 FROM "MPP_S 1"."T 2" ORDER BY c1 ASC NULLS LAST LIMIT 0::bigint
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Optimizer: Postgres-based planner
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Settings: gp_enable_minmax_optimization = 'off'
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(10 rows)
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EXPLAIN (VERBOSE, COSTS OFF)
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SELECT c1, c2 FROM mpp_ft2 order by c1 limit 0 offset null;
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QUERY PLAN
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--------------------------------------------------------------------------------------------------------------------------
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Limit
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Output: c1, c2
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-> Gather Motion 2:1 (slice1; segments: 2)
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Output: c1, c2
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Merge Key: c1
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-> Foreign Scan on public.mpp_ft2
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Output: c1, c2
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Remote SQL: SELECT c1, c2 FROM "MPP_S 1"."T 2" ORDER BY c1 ASC NULLS LAST LIMIT 0::bigint
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Optimizer: Postgres-based planner
1047+
Settings: gp_enable_minmax_optimization = 'off'
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(10 rows)
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1050+
EXPLAIN (VERBOSE, COSTS OFF)
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SELECT c1, c2 FROM mpp_ft2 order by c1 limit 3 offset null;
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QUERY PLAN
1053+
--------------------------------------------------------------------------------------------------------------------------
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Limit
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Output: c1, c2
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-> Gather Motion 2:1 (slice1; segments: 2)
1057+
Output: c1, c2
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Merge Key: c1
1059+
-> Foreign Scan on public.mpp_ft2
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Output: c1, c2
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Remote SQL: SELECT c1, c2 FROM "MPP_S 1"."T 2" ORDER BY c1 ASC NULLS LAST LIMIT 3::bigint
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Optimizer: Postgres-based planner
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Settings: gp_enable_minmax_optimization = 'off'
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(10 rows)
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EXPLAIN (VERBOSE, COSTS OFF)
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SELECT c1, c2 FROM mpp_ft2 order by c1 limit 3 offset 0;
1068+
QUERY PLAN
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-----------------------------------------------------------------------------------------------------------------------
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Limit
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Output: c1, c2
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-> Gather Motion 2:1 (slice1; segments: 2)
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Output: c1, c2
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Merge Key: c1
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-> Foreign Scan on public.mpp_ft2
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Output: c1, c2
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Remote SQL: SELECT c1, c2 FROM "MPP_S 1"."T 2" ORDER BY c1 ASC NULLS LAST LIMIT 3::bigint
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Optimizer: Postgres-based planner
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Settings: gp_enable_minmax_optimization = 'off'
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(10 rows)
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ALTER FOREIGN TABLE mpp_ft2 OPTIONS(set use_remote_estimate 'false');
9841083
-- Query with aggregates and limit clause together is NOT pushed down.
9851084
-- Because it's unsafe to do partial aggregate and limit in multiple remote servers.
9861085
EXPLAIN (VERBOSE, COSTS OFF)

‎contrib/postgres_fdw/sql/mpp_gp2pg_postgres_fdw.sql‎

Lines changed: 17 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -233,6 +233,23 @@ SELECT c1, c2 FROM mpp_ft2 order by c1 offset 2 limit 3;
233233
EXPLAIN VERBOSE
234234
SELECT c1, c2 FROM mpp_ft2 order by c1 offset 998;
235235
SELECT c1, c2 FROM mpp_ft2 order by c1 offset 998;
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237+
-- test LIMIT 0, OFFSET null/0
238+
ALTER FOREIGN TABLE mpp_ft2 OPTIONS(set use_remote_estimate 'true');
239+
EXPLAIN (VERBOSE, COSTS OFF)
240+
SELECT c1, c2 FROM mpp_ft2 order by c1 limit null offset 998;
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EXPLAIN (VERBOSE, COSTS OFF)
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SELECT c1, c2 FROM mpp_ft2 order by c1 limit all offset 998;
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EXPLAIN (VERBOSE, COSTS OFF)
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SELECT c1, c2 FROM mpp_ft2 order by c1 limit 0 offset 998;
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EXPLAIN (VERBOSE, COSTS OFF)
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SELECT c1, c2 FROM mpp_ft2 order by c1 limit 0 offset null;
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EXPLAIN (VERBOSE, COSTS OFF)
248+
SELECT c1, c2 FROM mpp_ft2 order by c1 limit 3 offset null;
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EXPLAIN (VERBOSE, COSTS OFF)
250+
SELECT c1, c2 FROM mpp_ft2 order by c1 limit 3 offset 0;
251+
ALTER FOREIGN TABLE mpp_ft2 OPTIONS(set use_remote_estimate 'false');
252+
236253
-- Query with aggregates and limit clause together is NOT pushed down.
237254
-- Because it's unsafe to do partial aggregate and limit in multiple remote servers.
238255
EXPLAIN (VERBOSE, COSTS OFF)

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