Hints (definition 1) on Hints (definition 2)

Transcription

1 Hints (definition 1) on Hints (definition 2) jonathanlewis.wordpress.com Who am I? Independent Consultant. 23+ years in IT 20+ using Oracle Strategy, Design, Review Briefings, Seminars Trouble-shooting jonathanlewis.wordpress.com One of the directors of the UKOUG Member of the Oak Table Network. Oracle Author of the year 2006 Select Editor s choice / 40 1

2 Highlights Guidelines for hinting Ignoring hints? Query Blocks - and getting lucky Precision matters Strategic hints 3 / 40 Guidelines for hinting 1. Don t 2. When you ignore #1 - be very careful 3. Try to stick to "strategic" (block level) hints 4. Check the outline for complete hinting 5. Test after every upgrade and patch 6. Test after every piece of associated DDL 4 / 40 2

3 Ignoring hints 1 Oracle MUST obey your hints Unless you ve got the syntax wrong Or you ve got the spelling wrong Or you ve got the context wrong Or the hint is illegal Or you have contradictory hints in one statement But the documentation on hints is poor There are 236 hints in (how many are documented?) select * from v$sql_hint 5 / 40 Ignoring hints 2 alter session set "_optimizer_ignore_hints"=true; /*+ IGNORE_OPTIM_EMBEDDED_HINTS */ And from one trace file I got: Trying or-expansion on query block SEL$1 (#0) USE_CONCAT hint was ignored. atom_hint=(@=221eaf74 err=0 resol=1 used=1 token=921 org=1 lvl=2 txt=use_concat (8) ) 6 / 40 3

4 Use of /*+ qb_name() */ (a) select /*+ qb_name(main) */ {list of columns} from t1, t3 where t1.n2 = 15 and exists (select /*+ qb_name(subq2) */ null from t2 where t2.n1 = 15 and t2.id = t1.id ) and t3.n1 = t1.n1 and t3.n2 = 15 and exists (select /*+ qb_name(subq4) */ null from t4 where t4.n1 = 15 and t4.id = t3.id ) 7 / 40 Use of /*+ qb_name() */ (b) select /*+ qb_name(main) no_unnest(@subq4) unnest(@subq2) leading(@sel$38f5b5f8 t1@main t3@main t2@subq2) no_push_subq(@subq4) index(@subq4 t4@subq4(id)) -- new style */ {list of columns} from t1, t3 where t1.n2 = 15 and exists (select /*+ qb_name(subq2) */... t2...) and t3.n1 = t1.n1 and t3.n2 = 15 and exists (select /*+ qb_name(subq4) */... t4...) 8 / 40 4

5 Use of /*+ qb_name() */ (c). Id Operation Name Rows 0 SELECT STATEMENT 1 1 FILTER 2 NESTED LOOPS SEMI 3 3 HASH JOIN TABLE ACCESS FULL T TABLE ACCESS FULL T TABLE ACCESS BY INDEX ROWID T2 1 7 INDEX UNIQUE SCAN T2_PK 1 8 TABLE ACCESS BY INDEX ROWID T4 1 9 INDEX UNIQUE SCAN T4_PK 1 Query Block Name / Object Alias (identified by operation id): 1 - SEL$38F5B5F8 4 - SEL$38F5B5F8 / T1@MAIN 5 - SEL$38F5B5F8 / T3@MAIN 6 - SEL$38F5B5F8 / T2@SUBQ2 7 - SEL$38F5B5F8 / T2@SUBQ2 8 - SUBQ4 / T4@SUBQ4 9 - SUBQ4 / T4@SUBQ4 9 / 40 Use of /*+ qb_name() */ (d) /*+ */ BEGIN_OUTLINE_DATA IGNORE_OPTIM_EMBEDDED_HINTS OPTIMIZER_FEATURES_ENABLE(' ') OPT_PARAM('_optimizer_cost_model' 'io') ALL_ROWS OUTLINE(@"SUBQ4") OUTLINE(@"MAIN") OUTLINE(@"SUBQ2") OUTLINE_LEAF(@"SUBQ4") OUTLINE_LEAF(@"SEL$38F5B5F8") UNNEST(@"SUBQ2") FULL(@"SEL$38F5B5F8" "T1"@"MAIN") FULL(@"SEL$38F5B5F8" "T3"@"MAIN") INDEX_RS_ASC(@"SEL$38F5B5F8" "T2"@"SUBQ2" ("T2"."ID")) LEADING(@"SEL$38F5B5F8" "T1"@"MAIN" "T3"@"MAIN" "T2"@"SUBQ2") USE_HASH(@"SEL$38F5B5F8" "T3"@"MAIN") USE_NL(@"SEL$38F5B5F8" "T2"@"SUBQ2") INDEX_RS_ASC(@"SUBQ4" "T4"@"SUBQ4" ("T4"."ID")) END_OUTLINE_DATA 10 / 40 5

6 Use of /*+ qb_name() */ (e) explain plan for... select * from table(dbms_xplan.display); select * from table(dbms_xplan.display(null,null,'{options}')); select * from table( dbms_xplan.display( null, null, 'alias outline -cost -bytes' ) ) ; 11 / 40 Precision matters (1a) select /*+ parallel(pt1,2) */ {list of columns} from pt1 where pt1.n1 = 5 and pt1.n2 = 10 and pt1.pt_group in (0,10) ; pt1 is list-partitioned on pt_group, with a local index on (n1, n2). The query visits two partitions. Id Operation Name Cost Pstart Pstop 0 SELECT STATEMENT 48 1 PARTITION LIST INLIST 48 KEY(I) KEY(I) *2 TABLE ACCESS BY LOCAL INDEX ROWID PT1 48 KEY(I) KEY(I) *3 INDEX RANGE SCAN PT1_I1 3 KEY(I) KEY(I) 12 / 40 6

7 Precision matters (1b) select /*+ full(pt1) parallel(pt1,2) */ {list of columns} from pt1 where pt1.n1 = 5 and pt1.n2 = 10 and pt1.pt_group in (0,10) ;. Id Operation Name Cost Pstart Pstop TQ IN-OUT PQ Distrib 0 SELECT STATEMENT 94 1 PX COORDINATOR 2 PX SEND QC (RANDOM) :TQ10 94 Q1,00 P->S QC (RAND) 3 PX BLOCK ITERATOR 94 KEY(I) KEY(I) Q1,00 PCWC * 4 TABLE ACCESS FULL PT1 94 KEY(I) KEY(I) Q1,00 PCWP 13 / 40 Precision matters (1c) select /*+ parallel(pt1,4) */ {list of columns} from pt1 where pt1.n1 = 5 and pt1.n2 = 10 and pt1.pt_group in (0,10) ;. Id Operation Name Cost Pstart Pstop TQ IN-OUT PQ Distrib 0 SELECT STATEMENT 47 1 PX COORDINATOR 2 PX SEND QC (RANDOM) :TQ10 47 Q1,00 P->S QC (RAND) 3 PX BLOCK ITERATOR 47 KEY(I) KEY(I) Q1,00 PCWC * 4 TABLE ACCESS FULL PT1 47 KEY(I) KEY(I) Q1,00 PCWP 14 / 40 7

8 Precision matters (2) NESTED LOOP TABLE ACCESS (FULL) OF T7 TABLE ACCESS BY INDEX ROWID T8 INDEX RANGE SCAN T8_IND_SECOND -- wrong index select /*+ index(t8 t8_ind_first) */ -- hint the right one HASH JOIN TABLE ACCESS (FULL) OF T7 TABLE ACCESS BY INDEX ROWID T8 INDEX FULL SCAN T8_IND_FIRST -- wrong method -- right index select /*+ index_rs_asc(t8 t8_ind_first) */ -- the right hint 15 / 40 Strategic Hints Shape the query, without forcing details: (no_)unnest (no_)push_subq (no_)merge (no_)push_pred driving_site() whether to transform subqueries or not when to run subqueries - early or late control handling of complex views how to join into non-mergeable views where to run a distributed query The hints about query block transformation are the safest ones 16 / 40 8

9 Feature Hints Becoming more common in 11g Switch off new features (matched to parameters) Also fairly safe to use For example: cursor_sharing_exact no_set_to_join no_use_hash_aggregation no_eliminate_join no_eliminate_oby overrides cursor_sharing = f / s disable set to join conversion disable hash aggregation disable join elimination disable order by elimination opt_param('parameter','value') 17 / 40 Unnesting (a) select outer.* from emp outer where outer.sal > ( select /*+ unnest redundant from 9i */ avg(inner.sal) from emp inner where inner.dept_no = outer.dept_no ) ; 18 / 40 9

10 Unnesting (b) select -- notional transformation (9i) outer.* from ( select dept_no, avg(sal) av_sal from emp group by dept_no ) inner, emp outer where outer.dept_no = inner.dept_no and outer.sal > inner.av_sal; 19 / 40 Unnesting (c) select -- investigated transformation (10g) outer.dept_no dept_no, outer.sal sal, outer.emp_no emp_no, outer.padding padding from test_user.emp inner, test_user.emp outer where inner.dept_no = outer.dept_no group by inner.dept_no, outer.rowid, outer.padding, outer.emp_no, outer.sal, outer.dept_no having outer.sal > avg(inner.sal) 20 / 40 10

11 Subquery pushdown (1) select t1.v1 from t1, t3 where t1.n2 = 15 /* and exists (select --+ no_unnest qb_name(subq2) null from t2 where t2.n1 = 15 and t2.id = t1.id ) */ and t3.n1 = t1.n1 and t3.n2 = 15 ; 21 / 40 Subquery pushdown (2) Execution Plan without subquery. Id Operation Name Rows Bytes Cost 0 SELECT STATEMENT * 1 HASH JOIN * 2 TABLE ACCESS FULL T * 3 TABLE ACCESS FULL T Execution Plan with subquery. Id Operation Name Rows Bytes Cost 0 SELECT STATEMENT * 1 FILTER * 2 HASH JOIN * 3 TABLE ACCESS FULL T * 4 TABLE ACCESS FULL T * 5 TABLE ACCESS BY INDEX ROWID T * 6 INDEX UNIQUE SCAN T2_PK / 40 11

12 Subquery pushdown (3) select /*+ ordered push_subq */ -- 9i use of hint /*+ ordered */ -- 10g use of hint /*+ */ -- 10g use of hint t1.v1 from t1, t3 where t1.n2 = 15 and exists (select --+ no_unnest qb_name(subq2) push_subq null from t2 where t2.n1 = 15 Alternative 10g use of hint. and t2.id = t1.id ) and t3.n1 = t1.n1 and t3.n2 = 15 ; 23 / 40 Subquery pushdown (4) Execution Plan without pushing. Id Operation Name Rows Bytes Cost(%CPU) Time 0 SELECT STATEMENT (2) 00:00:05 * 1 FILTER * 2 HASH JOIN (2) 00:00:03 * 3 TABLE ACCESS FULL T (2) 00:00:02 * 4 TABLE ACCESS FULL T (2) 00:00:02 * 5 TABLE ACCESS BY INDEX ROWID T (0) 00:00:01 * 6 INDEX UNIQUE SCAN T2_PK 1 1 (0) 00:00:01 Execution Plan pushed. Id Operation Name Rows Bytes Cost(%CPU) Time 0 SELECT STATEMENT (2) 00:00:03 * 1 HASH JOIN (2) 00:00:03 * 2 TABLE ACCESS FULL (filter) T (2) 00:00:02 * 3 TABLE ACCESS BY INDEX ROWID T (0) 00:00:01 * 4 INDEX UNIQUE SCAN T2_PK 1 1 (0) 00:00:01 * 5 TABLE ACCESS FULL T (2) 00:00:02 24 / 40 12

13 Non-mergeable views (a) Customers Orders Suppliers Products List all orders supplied to customers in Boston that contain products from suppliers in Dallas There may not be many of them, but there is no really efficient way to find them. Order_lines The only filters are at the ends of a five table join 25 / 40 Non-mergeable views (b) Customers Suppliers Orders Products Order_lines select /*+ ordered use_hash(prdsup) swap_join_inputs(prdsup) use_hash(cusord) swap_join_inputs(cusord) */ cusord.* from order_lines orl, ( select /*+ no_merge */ prd.id from products prd, suppliers sup where sup.location = 'Dallas' and prd.supp_id = sup.id ) prdsup, ( select /*+ no_merge */ ord.* from customers cus, orders ord where cus.location = 'Boston' and ord.cust_id = cus.id ) cusord where orl.order_id = cusord.id and prdsup.id = orl.product_id; 26 / 40 13

14 Non-mergeable views (c) 1 HASH JOIN 2 VIEW -- (1) hash this view 3 HASH JOIN 4 TABLE ACCESS (FULL) OF 'CUSTOMERS' 5 TABLE ACCESS (FULL) OF 'ORDERS' 6 HASH JOIN 7 VIEW -- (2) hash this view 8 HASH JOIN 9 TABLE ACCESS (FULL) OF 'SUPPLIERS' 10 TABLE ACCESS (FULL) OF 'PRODUCTS' 11 TABLE ACCESS (FULL) OF 'ORDER_LINES -- (3) scan this table Order lines are passed through the products/suppliers hash, then through the customers/orders hash. This could use a lot of memory. 27 / 40 Pushing Join Predicates (a) create or replace view v1 as select t2.id1, t2.id2, t3.small_vc, t3.padding from t2, t3 where t3.id1 = t2.id1 and t3.id2 = t2.id2 ; select /*+ push_pred(v1) */ t1.*, v1.* from t1, v1 where v1.id1(+) = t1.id1 and t1.n1 = 5 ; 28 / 40 14

15 Pushing Join Predicates (b) Plan before predicate pushing. Id Operation Name Rows Bytes Cost 0 SELECT STATEMENT * 1 HASH JOIN OUTER * 2 TABLE ACCESS FULL T VIEW V K 44 4 NESTED LOOPS K 44 5 TABLE ACCESS FULL T K 44 * 6 INDEX UNIQUE SCAN T2_PK 1 9 Predicate Information (identified by operation id): 1 - access("v1"."id1"(+)="t1"."id1") 2 - filter("t1"."n1"=5) 6 - access("t3"."id1"="t2"."id1" AND "T3"."ID2"="T2"."ID2") 29 / 40 Pushing Join Predicates (c) Plan with predicate pushing. Id Operation Name Rows Bytes Cost 0 SELECT STATEMENT NESTED LOOPS OUTER * 2 TABLE ACCESS FULL T VIEW PUSHED PREDICATE V NESTED LOOPS TABLE ACCESS BY INDEX ROWID T * 6 INDEX RANGE SCAN T3_PK 5 2 * 7 INDEX UNIQUE SCAN T2_PK 1 13 Predicate Information (identified by operation id): 2 - filter("t1"."n1"=5) 6 - access("t3"."id1"="t1"."id1") 7 - access("t2"."id1"="t1"."id1" AND "T3"."ID2"="T2"."ID2") filter("t3"."id1"="t2"."id1") 30 / 40 15

16 Distributed Effects create table dist_home {dist_away} as select rownum id, rpad(rownum,10) small_vc, rpad(rownum,200) large_vc from all_objects where rownum <= 2000; alter table dist_home {dist_away} add constraint dh_pk {da_pk} primary key (id); create public database link using 'd920'; 31 / 40 Distributed Effects select /*+ driving_site (dh) */ dh.small_vc, da.large_vc from dist_home dist_away@d920@loopback where dh.small_vc like '12%' and da.id = dh.id ; dh, da 32 / 40 16

17 Distributed Effects ID Row Source 0 SELECT STATEMENT (all_rows) 1 NESTED LOOPS 2 TABLE ACCESS(analyzed)DIST_HOME full 3 REMOTE SERIAL_FROM_REMOTE 3 D920@LOOPBACK -- object_node 3 SELECT "ID","LARGE_VC" -- other FROM "DIST_AWAY" "DA" WHERE "ID"=:1 -- read consistency? 33 / 40 Distributed Effects Change the filter predicate to select more data: dh.small_vc like '1%' ID Row Source 0 SELECT STATEMENT (all_rows) 1 HASH JOIN 2 TABLE ACCESS(analyzed)DIST_HOME full 3 REMOTE SERIAL_FROM_REMOTE 3 D920@LOOPBACK 3 SELECT "ID","LARGE_VC" FROM "DIST_AWAY" "DA" 34 / 40 17

18 Distributed Effects Tune the hash join by changing the driving site to the away d/b /*+ driving_site (da) */ ID Row Source 0 SELECT STATEMENT (all_rows)(remote) 1 HASH JOIN 2 REMOTE SERIAL_FROM_REMOTE 3 TABLE ACCESS(analyzed)DIST_AWAY full 2! -- where is this node? 3 D920@LOOPBACK -- this one knows who it is 2 SELECT "ID","SMALL_VC" FROM "DIST_HOME" "A2" WHERE "SMALL_VC" LIKE '1%' 35 / 40 Multi-table distributed joins select... from sales@d920@loopback sal, sites sit, products@d920@loopback prd where... SELECT STATEMENT Optimizer=ALL_ROWS HASH JOIN INDEX (FULL SCAN) OF 'SI_PK' (UNIQUE) REMOTE* SERIAL_FROM_REMOTE D920.JLCOMP.CO.UK@LOOPBACK SELECT "A1"."SALE_DATE",... "A2"."PROMOTED" FROM "SALES" "A1","PRODUCTS" "A2" WHERE / 40 18

19 Multi-table distributed joins 0 SELECT STATEMENT Optimizer=ALL_ROWS 1 NESTED LOOPS 2 NESTED LOOPS 3 REMOTE* D920.JLCOMP.CO.UK@LOOPBACK 4 INDEX (UNIQUE SCAN) OF 'SI_PK' (UNIQUE) 5 REMOTE* D920.JLCOMP.CO.UK@LOOPBACK 3 SELECT "SALE_DATE","SITE","PRODUCT","QTY","PROFIT" FROM "SALES" "SAL" 5 SELECT /*+ INDEX("PRD") USE_NL("PRD") */ "ID","PROMOTED" FROM "PRODUCTS" "PRD" WHERE "ID"= :1 AND "PROMOTED">TO_DATE(' :00:00', 'yyyy-mm-dd hh24:mi:ss') 37 / 40 How to use hints (1) Set the join order Ordered, leading (t1, t2, ) (10g version) But remember unnesting problems with ordered Join method for N-1 tables Use_nl, use_merge, use_hash + (no_)swap_join_inputs Access path for every table Index, no_index, hash, full, index_ffs etc. Average two hints per table to do it well (three if you use hash joins) 38 / 40 19

20 How to use hints (2) Block or force unnesting unnest, no_unnest - in the subquery Block or force early subqueries push_subq, no_push_subq change syntax in 10g Block or force merging merge, no_merge - two forms, with or w/o alias Block, or force, predicate pushing push_pred, no_push_pred (10g) Choosing the driving site for distributed Driving_site() 39 / 40 Summary Avoid micro-management hints Use query block names from 10g onwards Use "outline" to see the full set of hints Strategic hints can be useful and safe Feature hints can be useful and safe Keep testing on every upgrade 40 / 40 20