Can I ask Firebird to use index (with like on strings) in left join condition on other column? - sql

I have table structures:
journal_entries (id integer, account varchar(20), doc_date, date, amount numeric(15,4))
selected_accounts (account varchar(20), selection_id integer)
I can query this and SQL used indexes both on doc_date and on account:
select je.*
from journal_entries je
where je.doc_date>='01.01.2022' and
je.doc_date<='31.03.2022' and
(je.account like '23%' or
je.account like '24%')
But when I am filling the selected_accounts table with data:
23%, 1
24%, 1
And I am trying to use condition in left join:
select
from selected_accounts sa
left join journal_entries je on (
je.doc_date>='01.01.2022' and
je.doc_date<='31.03.2022' and
je.account like sa.account)
Then SQL is not using index on journal_entries.account data, it is using index on je.doc_date only.
Can I give some hints to optimizer or SQL engine that condition je.account like sa.account should use index on je.account?
I am using Firebird 3.1 and Firebird 2.1 but I guess, this issue is on other SQL databases as well.
I am gravitation towards the need to accept that I can not make optimal query with the condition in left join...
Question supplemented: I copied the plan (e.g. given by IBExpert) from the first query as the plan clause to the second query but the SQL engine reports:
index <index on journal_entries.account> cannot be used in the specified plan
So, there is something in my query that prevents the reference and use of journal_entries.account index.
Additional observation: In fact - my database have 1M journal entries in 2022Q1 (period specified in my examples), then the first (good) query reports less than 1M indexed record reads, but the second (bad) query reports 2*1M indexed record reads (indexed beacuse of the index on journal_entries.doc_date), so, this is even worse than the full read by doc_date and then just filter by selected_records entries.
One step forward: Thanks to #Damien_The_Unbeliever comment I made this test (sic! first string prefixed with %):
select je.*
from journal_entries je
where je.doc_date>='01.01.2022' and
je.doc_date<='31.03.2022' and
(je.account like '%23%' or
je.account like '24%')
And no more the je.account index is used and the number of reads increased. So - it seems to me that Firebird query engine/optimizer scans that string literals used in like conditions and decides on the possibility to use index on je.account.
So, maybe I can give some notice to Firebird (for my second/slow query) that I expect only post-%-fixed strings as select_accounts.account output? That would solve my issue in the case of Firebird engine.

It is not possible to optimize like in this case, because Firebird cannot know what values your column has. The fact somecolumn like '24%' can use an index is because Firebird will rewrite that expression to somecolumn starting with '24' (see also LIKE, specifically the note titled "About LIKE and the Optimizer"). It is not possible to do so with parameters or values obtained from columns.
In other words, the obvious solution to your problem is to have selected_accounts.account not be populated with '24%', but with '24', and using STARTING WITH in your join condition.
In the case the wildcard doesn't always occur, and some times you need an exact match, you could use something like je.account starting with replace(sa.account, '%', '') and je.account like sa.account. This solution assumes that % only ever occurs as the last character, and there is no _ wildcard used.

Related

difference b/w where column='' and column like '' in sql [duplicate]

This question skirts around what I'm wondering, but the answers don't exactly address it.
It would seem that in general '=' is faster than 'like' when using wildcards. This appears to be the conventional wisdom. However, lets suppose I have a column containing a limited number of different fixed, hardcoded, varchar identifiers, and I want to select all rows matching one of them:
select * from table where value like 'abc%'
and
select * from table where value = 'abcdefghijklmn'
'Like' should only need to test the first three chars to find a match, whereas '=' must compare the entire string. In this case it would seem to me that 'like' would have an advantage, all other things being equal.
This is intended as a general, academic question, and so should not matter which DB, but it arose using SQL Server 2005.
See https://web.archive.org/web/20150209022016/http://myitforum.com/cs2/blogs/jnelson/archive/2007/11/16/108354.aspx
Quote from there:
the rules for index usage with LIKE
are loosely like this:
If your filter criteria uses equals =
and the field is indexed, then most
likely it will use an INDEX/CLUSTERED
INDEX SEEK
If your filter criteria uses LIKE,
with no wildcards (like if you had a
parameter in a web report that COULD
have a % but you instead use the full
string), it is about as likely as #1
to use the index. The increased cost
is almost nothing.
If your filter criteria uses LIKE, but
with a wildcard at the beginning (as
in Name0 LIKE '%UTER') it's much less
likely to use the index, but it still
may at least perform an INDEX SCAN on
a full or partial range of the index.
HOWEVER, if your filter criteria uses
LIKE, but starts with a STRING FIRST
and has wildcards somewhere AFTER that
(as in Name0 LIKE 'COMP%ER'), then SQL
may just use an INDEX SEEK to quickly
find rows that have the same first
starting characters, and then look
through those rows for an exact match.
(Also keep in mind, the SQL engine
still might not use an index the way
you're expecting, depending on what
else is going on in your query and
what tables you're joining to. The
SQL engine reserves the right to
rewrite your query a little to get the
data in a way that it thinks is most
efficient and that may include an
INDEX SCAN instead of an INDEX SEEK)
It's a measureable difference.
Run the following:
Create Table #TempTester (id int, col1 varchar(20), value varchar(20))
go
INSERT INTO #TempTester (id, col1, value)
VALUES
(1, 'this is #1', 'abcdefghij')
GO
INSERT INTO #TempTester (id, col1, value)
VALUES
(2, 'this is #2', 'foob'),
(3, 'this is #3', 'abdefghic'),
(4, 'this is #4', 'other'),
(5, 'this is #5', 'zyx'),
(6, 'this is #6', 'zyx'),
(7, 'this is #7', 'zyx'),
(8, 'this is #8', 'klm'),
(9, 'this is #9', 'klm'),
(10, 'this is #10', 'zyx')
GO 10000
CREATE CLUSTERED INDEX ixId ON #TempTester(id)CREATE CLUSTERED INDEX ixId ON #TempTester(id)
CREATE NONCLUSTERED INDEX ixTesting ON #TempTester(value)
Then:
SET SHOWPLAN_XML ON
Then:
SELECT * FROM #TempTester WHERE value LIKE 'abc%'
SELECT * FROM #TempTester WHERE value = 'abcdefghij'
The resulting execution plan shows you that the cost of the first operation, the LIKE comparison, is about 10 times more expensive than the = comparison.
If you can use an = comparison, please do so.
You should also keep in mind that when using like, some sql flavors will ignore indexes, and that will kill performance. This is especially true if you don't use the "starts with" pattern like your example.
You should really look at the execution plan for the query and see what it's doing, guess as little as possible.
This being said, the "starts with" pattern can and is optimized in sql server. It will use the table index. EF 4.0 switched to like for StartsWith for this very reason.
If value is unindexed, both result in a table-scan. The performance difference in this scenario will be negligible.
If value is indexed, as Daniel points out in his comment, the = will result in an index lookup which is O(log N) performance. The LIKE will (most likely - depending on how selective it is) result in a partial scan of the index >= 'abc' and < 'abd' which will require more effort than the =.
Note that I'm talking SQL Server here - not all DBMSs will be nice with LIKE.
You are asking the wrong question. In databases is not the operator performance that matters, is always the SARGability of the expression, and the coverability of the overall query. Performance of the operator itself is largely irrelevant.
So, how do LIKE and = compare in terms of SARGability? LIKE, when used with an expression that does not start with a constant (eg. when used LIKE '%something') is by definition non-SARGabale. But does that make = or LIKE 'something%' SARGable? No. As with any question about SQL performance the answer does not lie with the query of the text, but with the schema deployed. These expression may be SARGable if an index exists to satisfy them.
So, truth be told, there are small differences between = and LIKE. But asking whether one operator or other operator is 'faster' in SQL is like asking 'What goes faster, a red car or a blue car?'. You should eb asking questions about the engine size and vechicle weight, not about the color... To approach questions about optimizing relational tables, the place to look is your indexes and your expressions in the WHERE clause (and other clauses, but it usually starts with the WHERE).
A personal example using mysql 5.5: I had an inner join between 2 tables, one of 3 million rows and one of 10 thousand rows.
When using a like on an index as below(no wildcards), it took about 30 seconds:
where login like '12345678'
using 'explain' I get:
When using an '=' on the same query, it took about 0.1 seconds:
where login ='12345678'
Using 'explain' I get:
As you can see, the like completely cancelled the index seek, so query took 300 times more time.
= is much faster than LIKE, even without wildcard. I tested on MySQL with 11GB of data and more than 100 million of records, the f_time column is indexed.
SELECT * FROM XXXXX WHERE f_time = '1621442261'
#took 0.00sec and return 330 records
SELECT * FROM XXXXX WHERE f_time LIKE '1621442261'
#took 44.71sec and return 330 records
Besides all the answers, there this to consider:
'like' is case insensitive, so every character needs to be compared twice, whereas the '=' only compares once for identical characters.
This issue arises with or without indexes.
Maybe you are looking about Full Text Search.
In contrast to full-text search, the LIKE Transact-SQL predicate works on
character patterns only. Also, you cannot use the LIKE predicate to
query formatted binary data. Furthermore, a LIKE query against a large
amount of unstructured text data is much slower than an equivalent
full-text query against the same data. A LIKE query against millions
of rows of text data can take minutes to return; whereas a full-text
query can take only seconds or less against the same data, depending
on the number of rows that are returned.
I was working with a huge database that has more then 400M records and I put LIKE in search query. Here is the final results.
There were three tables tb1, tb2 and tb3. When I use EQUAL for in all tables QUERY the response time was 193ms. and when I put LIKE in one of he table the response time was 19.22 sec. and for all table LIKE response time was 112 Sec

Oracle CONTAINS() not returning results for numbers

So I have this table with a full text indexed column "value". The field contains some number strings in it, which are correctly returned when i use a query like so:
select value
from mytable
where value like '17946234';
Trouble is, it's incredibly slow because there are a lot of rows in this table, but when i use the CONTAINS operator I get no results:
select value
from mytable
where CONTAINS ( value, '17946234',1)>0
Anyone got any thoughts?
Unfortunately, I'm not an Oracle dude, and the same query works fine in SQL Server. I feel like it must be a stoplist or something with the Oracle Lexer that I can change, but not really sure how.
This could be due to INDEXTYPE IS CTXSYS.CONTEXT in general, or the index not having been updated after the looked for records where added (CONTEXT type indexes are not transactional, whilst CTXCAT type ones are).
However, if you did not accidentally lose the wildcard in your statement (In other words: If no wildcard is required indeed.), you could just query
select value from mytable where value = '17946234';
which could possibly be backed by an ordinary index. (Depending on your specific data distribution and the queries run, an index might not help query performance.)
Otherwise
select value from mytable where instr(value, '17946234') > 0;
might be sufficient.

Does SQL performance degrade as the number elements in an "IN" clause increases?

I have a query like this,
SELECT Name FROM Customers WHERE Id IN (1,4,3,6,7)
There might be millions of customers in the DataBase. Will there be an efficiency problem with this query ? When the number of Ids inside IN statement are more ? If so, Why and Any workaround ?
I Use SQLServer. Below is my table Structure
Id|Name|Designation
Id is the primary key -non clustered index.
This query is as basic as it can get.
If you need to find the name of 5 customers, there is simply no other sane way of writing it.
It will perform well if you have an index on ID. The performance is almost instantaneous, directly related to the number of items in the IN clause.
If you don't it will scan the table, and the performance becomes directly related to the number of records in the table.
Assuming you have properly indexed the Id column, there should be no problem. That is the correct method, and if it does not work, you need a new database. (Millions shouldn't be an issue with most regular pieces of software; if you make it to multiple billions you might need to investigate clustered databases).
If you execute the following query:
select * from sys.objects where object_id in (
1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,
31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,
58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,
85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,
109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,
130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,
151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,256,257,258,259,260,261,262,263,264,265,266,267,268,269,270,271,272,273,274,275,276,277,278,279,280,281,282,283,284,285,286,287,288,289,290,291,292,293,294,295,296,297,298,299,300,301,302,303,304,305,306,307,308,309,310,311,312,313,314,315,316,317,318,319,320,321,322,323,324,325,326,327,328,329,330,331,332,333,334,335,336,337,338,339,340,341,342,343,344,345,346,347,348,349,350,351,352,353,354,355,356,357,358,359,360,361,362,363,364,365,366,367,368,369,370,371,372,373,374,375,376,377,378,379,380,381,382,383,384,385,386,387,388,389,390,391,392,393,394,395,396,397,398,399,400,401,402,403,404,405,406,407,408,409,410,411,412,413,414,415,416,417,418,419,420,421,422,423,424,425,426,427,428,429,430,431,432,433,434,435,436,437,438,439,440,441,442,443,444,445,446,447,448,449,450,451,452,453,454,455,456,457,458,459,460,461,462,463,464,465,466,467,468,469,470,471,472,473,474,475,476,477,478,479,480,481,482,483,484,485,486,487,488,489,490,491,492,493,494,495,496,497,498,499,500,501,502,503,504,505,506,507,508,509,510,511,512,513,514,515,516,517,518,519,520,521,522,523,524,525,526,527,528,529,530,531,532,533,534,535,536,537,538,539,540,541,542,543,544,545,546,547,548,549,550,551,552,553,554,555,556,557,558,559,560,561,562,563,564,565,566,567,568,569,570,571,572,573,574,575,576,577,578,579,580,581,582,583,584,585,586,587,588,589,590,591,592,593,594,595,596,597,598,599,600,601,602,603,604,605,606,607,608,609,610,611,612,613,614,615,616,617,618,619,620,621,622,623,624,625,626,627,628,629,630,631,632,633,634,635,636,637,638,639,640,641,642,643,644,645,646,647,648,649,650,651,652,653,654,655,656,657,658,659,660,661,662,663,664,665,666,667,668,669,670,671,672,673,674,675,676,677,678,679,680,681,682,683,684,685,686,687,688,689,690,691,692,693,694,695,696,697,698,699,700,701,702,703,704,705,706,707,708,709,710,711,712,713,714,715,716,717,718,719,720,721,722,723,724,725,726,727,728,729,730,731,732,733,734,735,736,737,738,739,740,741,742,743,744,745,746,747,748,749,750,751,752,753,754,755,756,757,758,759,760,761,762,763,764,765,766,767,768,769,770,771,772,773,774,775,776,777,778,779,780,781,782,783,784,785,786,787,788,789,790,791,792,793,794,795,796,797,798,799,800,801,802,803,804,805,806,807,808,809,810,811,812,813,814,815,816,817,818,819,820,821,822,823,824,825,826,827,828,829,830,831,832,833,834,835,836,837,838,839,840,841,842,843,844,845,846,847,848,849,850,851,852,853,854,855,856,857,858,859,860,861,862,863,864,865,866,867,868,869,870,871,872,873,874,875,876,877,878,879,880,881,882,883,884,885,886,887,888,889,890,891,892,893,894,895,896,897,898,899,900,901,902,903,904,905,906,907,908,909,910,911,912,913,914,915,916,917,918,919,920,921,922,923,924,925,926,927,928,929,930,931,932,933,934,935,936,937,938,939,940,941,942,943,944,945,946,947,948,949,950,951,952,953,954,955,956,957,958,959,960,961,962,963,964,965,966,967,968,969,970,971,972,973,974,975,976,977,978,979,980,981,982,983,984,985,986,987,988,989,990,991,992,993,994,995,996,997,998,999,1000,1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1011,1012,1013,1014,1015,1016,1017,1018,1019,1020,1021,1022,1023,1024,1025,1026,1027,1028,1029,1030,1031,1032,1033,1034,1035,1036,1037,1038,1039,1040,1041,1042,1043,1044,1045,1046,1047,1048,1049,1050,1051,1052,1053,1054,1055,1056,1057,1058,1059,1060,1061,1062,1063,1064,1065,1066,1067,1068,1069,1070,1071,1072,1073,1074,1075,1076,1077,1078,1079,1080,1081,1082,1083,1084,1085,1086,1087,1088,1089,1090,1091,1092,1093,1094,1095,1096,1097,1098,1099,1100,1101,1102,1103,1104,1105,1106,1107,1108,1109,1110,1111,1112,1113,1114,1115,1116,1117,1118,1119,1120,1121,1122,1123,1124,1125,1126,1127,1128,1129,1130,1131,1132,1133,1134,1135,1136,1137,1138,1139,1140,1141,1142,1143,1144,1145,1146,1147,1148,1149,1150,1151,1152,1153,1154,1155,1156,1157,1158,1159,1160,1161,1162,1163,1164,1165,1166,1167,1168,1169,1170,1171,1172,1173,1174,1175,1176,1177,1178,1179,1180,1181,1182,1183,1184,1185,1186,1187,1188,1189,1190,1191,1192,1193,1194,1195,1196,1197,1198,1199,1200,1201,1202,1203,1204,1205,1206,1207,1208,1209,1210,1211,1212,1213,1214,1215,1216,1217,1218,1219,1220,1221,1222,1223,1224,1225,1226,1227,1228,1229,1230,1231,1232,1233,1234,1235,1236,1237,1238,1239,1240,1241,1242,1243,1244,1245,1246,1247,1248,1249,1250,1251,1252,1253,1254,1255,1256,1257,1258,1259,1260,1261,1262,1263,1264,1265,1266,1267,1268,1269,1270,1271,1272,1273,1274,1275,1276,1277,1278,1279,1280,1281,1282,1283,1284,1285,1286,1287,1288,1289,1290,1291,1292,1293,1294,1295,1296,1297,1298,1299,1300,1301,1302,1303,1304,1305,1306,1307,1308,1309,1310,1311,1312,1313,1314,1315,1316,1317,1318,1319,1320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5761,5762,5763,5764,5765,5766,5767,5768,5769,5770,5771,5772,5773,5774,5775,5776)
(I'm not going to break up all the lines).
In the resulting query, approximately 5% of the cost of the query is taken up with a constant scan (which is effectively turning all of those numbers into a temp table internally and that table is then passed to a join operator).
But, this is a remarkably simple query overall. For any more complex query, I'd expect that the cost, as a percentage, will go down (since I expect the absolute cost to remain the same)
I know this isn't the question that was asked, but, say your list of IDs came from another query:
SELECT Name FROM Customers WHERE Id IN (SELECT ID FROM X WHERE X.FIELD = COND)
Then this is cause to rewrite your query using EXISTS:
SELECT Name
FROM CUSTOMERS
WHERE EXISTS
(
SELECT *
FROM X
WHERE X.FIELD = COND
AND X.ID = CUSTOMERS.ID
);
This is efficient because EXISTS gives more opportunity for the optimizer to determine an efficient execution path, whereas IN forces the subquery to be fully evaluated.
The query you specified didn't have a subsquery. It just has a list of constants which has little opportunity to be further optimized. As is, you have to do with the best you got, i.e. index the ID column as recommended by #zebediah49.

IN vs OR of Oracle, which faster?

I'm developing an application which processes many data in Oracle database.
In some case, I have to get many object based on a given list of conditions, and I use SELECT ...FROM.. WHERE... IN..., but the IN expression just accepts a list whose size is maximum 1,000 items.
So I use OR expression instead, but as I observe -- perhaps this query (using OR) is slower than IN (with the same list of condition). Is it right? And if so, how to improve the speed of query?
IN is preferable to OR -- OR is a notoriously bad performer, and can cause other issues that would require using parenthesis in complex queries.
Better option than either IN or OR, is to join to a table containing the values you want (or don't want). This table for comparison can be derived, temporary, or already existing in your schema.
In this scenario I would do this:
Create a one column global temporary table
Populate this table with your list from the external source (and quickly - another whole discussion)
Do your query by joining the temporary table to the other table (consider dynamic sampling as the temporary table will not have good statistics)
This means you can leave the sort to the database and write a simple query.
Oracle internally converts IN lists to lists of ORs anyway so there should really be no performance differences. The only difference is that Oracle has to transform INs but has longer strings to parse if you supply ORs yourself.
Here is how you test that.
CREATE TABLE my_test (id NUMBER);
SELECT 1
FROM my_test
WHERE id IN (1,2,3,4,5,6,7,8,9,10,
21,22,23,24,25,26,27,28,29,30,
31,32,33,34,35,36,37,38,39,40,
41,42,43,44,45,46,47,48,49,50,
51,52,53,54,55,56,57,58,59,60,
61,62,63,64,65,66,67,68,69,70,
71,72,73,74,75,76,77,78,79,80,
81,82,83,84,85,86,87,88,89,90,
91,92,93,94,95,96,97,98,99,100
);
SELECT sql_text, hash_value
FROM v$sql
WHERE sql_text LIKE '%my_test%';
SELECT operation, options, filter_predicates
FROM v$sql_plan
WHERE hash_value = '1181594990'; -- hash_value from previous query
SELECT STATEMENT
TABLE ACCESS FULL ("ID"=1 OR "ID"=2 OR "ID"=3 OR "ID"=4 OR "ID"=5
OR "ID"=6 OR "ID"=7 OR "ID"=8 OR "ID"=9 OR "ID"=10 OR "ID"=21 OR
"ID"=22 OR "ID"=23 OR "ID"=24 OR "ID"=25 OR "ID"=26 OR "ID"=27 OR
"ID"=28 OR "ID"=29 OR "ID"=30 OR "ID"=31 OR "ID"=32 OR "ID"=33 OR
"ID"=34 OR "ID"=35 OR "ID"=36 OR "ID"=37 OR "ID"=38 OR "ID"=39 OR
"ID"=40 OR "ID"=41 OR "ID"=42 OR "ID"=43 OR "ID"=44 OR "ID"=45 OR
"ID"=46 OR "ID"=47 OR "ID"=48 OR "ID"=49 OR "ID"=50 OR "ID"=51 OR
"ID"=52 OR "ID"=53 OR "ID"=54 OR "ID"=55 OR "ID"=56 OR "ID"=57 OR
"ID"=58 OR "ID"=59 OR "ID"=60 OR "ID"=61 OR "ID"=62 OR "ID"=63 OR
"ID"=64 OR "ID"=65 OR "ID"=66 OR "ID"=67 OR "ID"=68 OR "ID"=69 OR
"ID"=70 OR "ID"=71 OR "ID"=72 OR "ID"=73 OR "ID"=74 OR "ID"=75 OR
"ID"=76 OR "ID"=77 OR "ID"=78 OR "ID"=79 OR "ID"=80 OR "ID"=81 OR
"ID"=82 OR "ID"=83 OR "ID"=84 OR "ID"=85 OR "ID"=86 OR "ID"=87 OR
"ID"=88 OR "ID"=89 OR "ID"=90 OR "ID"=91 OR "ID"=92 OR "ID"=93 OR
"ID"=94 OR "ID"=95 OR "ID"=96 OR "ID"=97 OR "ID"=98 OR "ID"=99 OR
"ID"=100)
I would question the whole approach. The client of the SP has to send 100000 IDs. Where does the client get those IDs from? Sending such a large number of ID as the parameter of the proc is going to cost significantly anyway.
If you create the table with a primary key:
CREATE TABLE my_test (id NUMBER,
CONSTRAINT PK PRIMARY KEY (id));
and go through the same SELECTs to run the query with the multiple IN values, followed by retrieving the execution plan via hash value, what you get is:
SELECT STATEMENT
INLIST ITERATOR
INDEX RANGE SCAN
This seems to imply that when you have an IN list and are using this with a PK column, Oracle keeps the list internally as an "INLIST" because it is more efficient to process this, rather than converting it to ORs as in the case of an un-indexed table.
I was using Oracle 10gR2 above.

FreeText Query is slow - includes TOP and Order By

The Product table has 700K records in it. The query:
SELECT TOP 1 ID,
Name
FROM Product
WHERE contains(Name, '"White Dress"')
ORDER BY DateMadeNew desc
takes about 1 minute to run. There is an non-clustered index on DateMadeNew and FreeText index on Name.
If I remove TOP 1 or Order By - it takes less then 1 second to run.
Here is the link to execution plan.
http://screencast.com/t/ZDczMzg5N
Looks like FullTextMatch has over 400K executions. Why is this happening? How can it be made faster?
UPDATE 5/3/2010
Looks like cardinality is out of whack on multi word FreeText searches:
Optimizer estimates that there are 28K records matching 'White Dress', while in reality there is only 1.
http://screencast.com/t/NjM3ZjE4NjAt
If I replace 'White Dress' with 'White', estimated number is '27,951', while actual number is '28,487' which is a lot better.
It seems like Optimizer is using only the first word in phrase being searched for cardinality.
Looks like FullTextMatch has over 400K executions. Why is this happening?
Since you have an index combined with TOP 1, optimizer thinks that it will be better to traverse the index, checking each record for the entry.
How can it be made faster?
If updating the statistics does not help, try adding a hint to your query:
SELECT TOP 1 *
FROM product pt
WHERE CONTAINS(name, '"test1"')
ORDER BY
datemadenew DESC
OPTION (HASH JOIN)
This will force the engine to use a HASH JOIN algorithm to join your table and the output of the fulltext query.
Fulltext query is regarded as a remote source returning the set of values indexed by KEY INDEX provided in the FULLTEXT INDEX definition.
Update:
If your ORM uses parametrized queries, you can create a plan guide.
Use Profiler to intercept the query that the ORM sends verbatim
Generate a correct plan in SSMS using hints and save it as XML
Use sp_create_plan_guide with an OPTION USE PLAN to force the optimizer always use this plan.
Edit
From http://technet.microsoft.com/en-us/library/cc721269.aspx#_Toc202506240
The most important thing is that the
correct join type is picked for
full-text query. Cardinality
estimation on the FulltextMatch STVF
is very important for the right plan.
So the first thing to check is the
FulltextMatch cardinality estimation.
This is the estimated number of hits
in the index for the full-text search
string. For example, in the query in
Figure 3 this should be close to the
number of documents containing the
term ‘word’. In most cases it should
be very accurate but if the estimate
was off by a long way, you could
generate bad plans. The estimation for
single terms is normally very good,
but estimating multiple terms such as
phrases or AND queries is more complex
since it is not possible to know what
the intersection of terms in the index
will be based on the frequency of the
terms in the index. If the cardinality
estimation is good, a bad plan
probably is caused by the query
optimizer cost model. The only way to
fix the plan issue is to use a query
hint to force a certain kind of join
or OPTIMIZE FOR.
So it simply cannot know from the information it stores whether the 2 search terms together are likely to be quite independent or commonly found together. Maybe you should have 2 separate procedures one for single word queries that you let the optimiser do its stuff on and one for multi word procedures that you force a "good enough" plan on (sys.dm_fts_index_keywords might help if you don't want a one size fits all plan).
NB: Your single word procedure would likely need the WITH RECOMPILE option looking at this bit of the article.
In SQL Server 2008 full-text search we have the ability to alter the plan that is generated based on a cardinality estimation of the search term used. If the query plan is fixed (as it is in a parameterized query inside a stored procedure), this step does not take place. Therefore, the compiled plan always serves this query, even if this plan is not ideal for a given search term.
Original Answer
Your new plan still looks pretty bad though. It looks like it is only returning 1 row from the full text query part but scanning all 770159 rows in the Product table.
How does this perform?
CREATE TABLE #tempResults
(
ID int primary key,
Name varchar(200),
DateMadeNew datetime
)
INSERT INTO #tempResults
SELECT
ID, Name, DateMadeNew
FROM Product
WHERE contains(Name, '"White Dress"')
SELECT TOP 1
*
FROM #tempResults
ORDER BY DateMadeNew desc
I can't see the linked execution plan, network police are blocking that, so this is just a guess...
if it is running fast without the TOP and ORDER BY, try doing this:
SELECT TOP 1
*
FROM (SELECT
ID, Name, DateMadeNew
FROM Product
WHERE contains(Name, '"White Dress"')
) dt
ORDER BY DateMadeNew desc
A couple of thoughts on this one:
1) Have you updated the statistics on the Product table? It would be useful to see the estimates and actual number of rows on the operations there too.
2) What version of SQL Server are you using? I had a similar issue with SQL Server 2008 that turned out to be nothing more than not having Service Pack 1 installed. Install SP1 and a FreeText query that was taking a couple of minutes (due to a huge number of actual executions against actual) went down to taking a second.
I had the same problem earlier.
The performance depends on which unique index you choose for full text indexing.
My table has two unique columns - ID and article_number.
The query:
select top 50 id, article_number, name, ...
from ARTICLE
CONTAINS(*,'"BLACK*" AND "WHITE*"')
ORDER BY ARTICLE_NUMBER
If the full text index is connected to ID then it is slow depending on the searched words.
If the full text index is connected to ARTICLE_NUMBER UNIQUE index then it was always fast.
I have better solution.
I. Let's first overview proposed solutions as they also may be used in some cases:
OPTION (HASH JOIN) - is not good as you may get error "Query processor could not produce a query plan because of the hints defined in this query. Resubmit the query without specifying any hints and without using SET FORCEPLAN."
SELECT TOP 1 * FROM (ORIGINAL_SELECT) ORDER BY ... - is not good, when you need to use paginating results from you ORIGINAL_SELECT
sp_create_plan_guide - is not good, as to use plan_guide you have to save plan for specific sql statement, this won't work for dynamic sql statements (e.g. generated by ORM)
II. My Solution contains of two parts
1. Self join table used for Full Text search
2. Use MS SQL HASH Join Hints MSDN Join Hints
Your SQL :
SELECT TOP 1 ID, Name FROM Product WHERE contains(Name, '"White Dress"')
ORDER BY DateMadeNew desc
Should be rewritten as :
SELECT TOP 1 p.ID, p.Name FROM Product p INNER HASH JOIN Product fts ON fts.ID = p.ID
WHERE contains(fts.Name, '"White Dress"')
ORDER BY p.DateMadeNew desc
If you are using NHibernate with/without Castle Active Records, I've replied in post how to write interceptor to modify your query to replace INNER JOIN by INNER HASH JOIN