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  • What are some best practises and "rules of thumb" for creating database indexes?

    - by Ash
    I have an app, which cycles through a huge number of records in a database table and performs a number of SQL and .Net operations on records within that database (currently I am using Castle.ActiveRecord on PostgreSQL). I added some basic btree indexes on a couple of the feilds, and as you would expect, the peformance of the SQL operations increased substantially. Wanting to make the most of dbms performance I want to make some better educated choices about what I should index on all my projects. I understand that there is a detrement to performance when doing inserts (as the database needs to update the index, as well as the data), but what suggestions and best practices should I consider with creating database indexes? How do I best select the feilds/combination of fields for a set of database indexes (rules of thumb)? Also, how do I best select which index to use as a clustered index? And when it comes to the access method, under what conditions should I use a btree over a hash or a gist or a gin (what are they anyway?).

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  • Help on understanding multiple columns on an index?

    - by Xaisoft
    Assume I have a table called "table" and I have 3 columns, a, b, and c. What does it mean to have a non-clustered index on columns a,b? Is a nonclustered index on columns a,b the same as a nonclustered index on columns b,a? (Note the order). Also, Is a nonclustered index on column a the same as a nonclustered index on a,c? I was looking at the website sqlserver performance and they had these dmv scripts where it would tell you if you had overlapping indexes and I believe it was saying that having an index on a is the same as a,b, so it is redundant. Is this true about indexes? One last question is why is the clustered index put on the primary key. Most of the time the primary key is not queried against, so shouldn't the clustered index be on the most queried column. I am probably missing something here like having it on the primary key speeds up joins? Great explanations. Should I turn this into a wiki and change the title index explanation?

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  • How do you efficiently bulk index lookups?

    - by Liron Shapira
    I have these entity kinds: Molecule Atom MoleculeAtom Given a list(molecule_ids) whose lengths is in the hundreds, I need to get a dict of the form {molecule_id: list(atom_ids)}. Likewise, given a list(atom_ids) whose length is in the hunreds, I need to get a dict of the form {atom_id: list(molecule_ids)}. Both of these bulk lookups need to happen really fast. Right now I'm doing something like: atom_ids_by_molecule_id = {} for molecule_id in molecule_ids: moleculeatoms = MoleculeAtom.all().filter('molecule =', db.Key.from_path('molecule', molecule_id)).fetch(1000) atom_ids_by_molecule_id[molecule_id] = [ MoleculeAtom.atom.get_value_for_datastore(ma).id() for ma in moleculeatoms ] Like I said, len(molecule_ids) is in the hundreds. I need to do this kind of bulk index lookup on almost every single request, and I need it to be FAST, and right now it's too slow. Ideas: Will using a Molecule.atoms ListProperty do what I need? Consider that I am storing additional data on the MoleculeAtom node, and remember it's equally important for me to do the lookup in the molecule-atom and atom-molecule directions. Caching? I tried memcaching lists of atom IDs keyed by molecule ID, but I have tons of atoms and molecules, and the cache can't fit it. How about denormalizing the data by creating a new entity kind whose key name is a molecule ID and whose value is a list of atom IDs? The idea is, calling db.get on 500 keys is probably faster than looping through 500 fetches with filters, right?

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  • SQLAlchemy custom sorting algorithms when using SQL indexes

    - by David M
    Is it possible to write custom collation functions with indexes in SQLAlchemy? SQLite for example allows specifying the sorting function at a C level as sqlite3_create_collation(). An implementation of some of the Unicode collation algorithm has been provided by James Tauber here, which for example sorts all the "a"'s close together whether they have accents on them or not. Other examples of why this might be useful is for different alphabet orders (languages other than English) and sorting numeric values (sorting 10 after 9 rather than codepoint order.) Is this possible in SQLAlchemy? If not, is it supported by the pysqlite3 or MySQLdb modules, or for any other SQL database modules supported by python for that matter? Any information would be greatly appreciated.

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  • How to optimize this SQL query for a rectangular region?

    - by Andrew B.
    I'm trying to optimize the following query, but it's not clear to me what index or indexes would be best. I'm storing tiles in a two-dimensional plane and querying for rectangular regions of that plane. The table has, for the purposes of this question, the following columns: id: a primary key integer world_id: an integer foreign key which acts as a namespace for a subset of tiles tileY: the Y-coordinate integer tileX: the X-coordinate integer value: the contents of this tile, a varchar if it matters. I have the following indexes: "ywot_tile_pkey" PRIMARY KEY, btree (id) "ywot_tile_world_id_key" UNIQUE, btree (world_id, "tileY", "tileX") "ywot_tile_world_id" btree (world_id) And this is the query I'm trying to optimize: ywot=> EXPLAIN ANALYZE SELECT * FROM "ywot_tile" WHERE ("world_id" = 27685 AND "tileY" <= 6 AND "tileX" <= 9 AND "tileX" >= -2 AND "tileY" >= -1 ); QUERY PLAN ------------------------------------------------------------------------------------------------------------------------------------------- Bitmap Heap Scan on ywot_tile (cost=11384.13..149421.27 rows=65989 width=168) (actual time=79.646..80.075 rows=96 loops=1) Recheck Cond: ((world_id = 27685) AND ("tileY" <= 6) AND ("tileY" >= (-1)) AND ("tileX" <= 9) AND ("tileX" >= (-2))) -> Bitmap Index Scan on ywot_tile_world_id_key (cost=0.00..11367.63 rows=65989 width=0) (actual time=79.615..79.615 rows=125 loops=1) Index Cond: ((world_id = 27685) AND ("tileY" <= 6) AND ("tileY" >= (-1)) AND ("tileX" <= 9) AND ("tileX" >= (-2))) Total runtime: 80.194 ms So the world is fixed, and we are querying for a rectangular region of tiles. Some more information that might be relevant: All the tiles for a queried region may or may not be present The height and width of a queried rectangle are typically about 10x10-20x20 For any given (world, X) or (world, Y) pair, there may be an unbounded number of matching tiles, but the worst case is currently around 10,000, and typically there are far fewer. New tiles are created far less frequently than existing ones are updated (changing the 'value'), and that itself is far less frequent that just reading as in the query above. The only thing I can think of would be to index on (world, X) and (world, Y). My guess is that the database would be able to take those two sets and intersect them. The problem is that there is a potentially unbounded number of matches for either for either of those. Is there some other kind of index that would be more appropriate?

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  • Should I specify both INDEX and UNIQUE INDEX?

    - by Matt Huggins
    On one of my PostgreSQL tables, I have a set of two fields that will be defined as being unique in the table, but will also both be used together when selecting data. Given this, do I only need to define a UNIQUE INDEX, or should I specify an INDEX in addition to the UNIQUE INDEX? This? CREATE UNIQUE INDEX mytable_col1_col2_idx ON mytable (col1, col2); Or this? CREATE UNIQUE INDEX mytable_col1_col2_uidx ON mytable (col1, col2); CREATE INDEX mytable_col1_col2_idx ON mytable (col1, col2);

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  • How to recognize what indexes are not used?

    - by tomaszs
    I have a table in MySQL with 7 indexes, most of them are on more than one column. I think here is too much indexes. Is there any way to get statistics of what indexes are used more by all thousands of queries to this database and what are less worthy so I know what index to consider to remove in first place?

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  • In SQL, in what situation do we want to Index a field in a table, or 2 fields in a table at the same

    - by Jian Lin
    In SQL, it is obvious that whenever we want to do a search on millions of record, say CustomerID in a Transactios table, then we want to add an index for CustomerID. Is another situation we want to add an index to a field when we need to do inner join or outer join using that field as a criteria? Such as Inner join on t1.custumerID = t2.customerID. Then if we don't have an index on customerID on both tables, we are looking at O(n^2) because we need to loop through the 2 tables sequentially. If we have index on customerID on both tables, then it becomes O( (log n) ^ 2 ) and it is much faster. Any other situation where we want to add an index to a field in a table? What about adding index for 2 fields combined in a table. That is, one index, for 2 fields together?

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  • Pruning data for better viewing on loglog graph - Matlab

    - by Geodesic
    Hi Guys, just wondering if anyone has any ideas about an issue I'm having. I have a fair amount of data that needs to be displayed on one graph. Two theoretical lines that are bold and solid are displayed on top, then 10 experimental data sets that converge to these lines are graphed, each using a different identifier (eg the + or o or a square etc). These graphs are on a log scale that goes up to 1e6. The first few decades of the graph (< 1e3) look fine, but as all the datasets converge ( 1e3) it's really difficult to see what data is what. There's over 1000 data points points per decade which I can prune linearly to an extent, but if I do this too much the lower end of the graph will suffer in resolution. What I'd like to do is prune logarithmically, strongest at the high end, working back to 0. My question is: how can I get a logarithmically scaled index vector rather than a linear one? My initial assumption was that as my data is lenear I could just use a linear index to prune, which lead to something like this (but for all decades): //%grab indicies per decade ind12 = find(y >= 1e1 & y <= 1e2); indlow = find(y < 1e2); indhigh = find(y > 1e4); ind23 = find(y >+ 1e2 & y <= 1e3); ind34 = find(y >+ 1e3 & y <= 1e4); //%We want ind12 indexes in this decade, find spacing tot23 = round(length(ind23)/length(ind12)); tot34 = round(length(ind34)/length(ind12)); //%grab ones to keep ind23keep = ind23(1):tot23:ind23(end); ind34keep = ind34(1):tot34:ind34(end); indnew = [indlow' ind23keep ind34keep indhigh']; loglog(x(indnew), y(indnew)); But this causes the prune to behave in a jumpy fashion obviously. Each decade has the number of points that I'd like, but as it's a linear distribution, the points tend to be clumped at the high end of the decade on the log scale. Any ideas on how I can do this?

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  • Lucene.NET - Find documents that do not contain a specified field

    - by Brandon
    Let's say I have 2 instance of a class called 'Animal'. Animal has 3 fields: Name, Age, and Type The name field is nullable, so before I insert an instance of Animal as a Lucene indexed document, I check if Animal.Name == null, and if it does, I do not insert it as a field in my document. If I were to retrieve all animals, I would see that the Name field does not exist and I can set its value to null. However, there may be situations where I want to say "Get me all animals that do not have a name specified yet." In this situation I want to retrieve all Lucene.NET documents from my animal index that do not contain the Name field. Is there an easy way to do this with Lucene.NET? I want to stay away from having to perform some sort of hack to check if my name field has a value of 'null'.

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  • List of all index & index columns in SQL Server DB

    - by Anton Gogolev
    How do I get a list of all index & index columns in SQL Server 2005+? The closest I could get is: select s.name, t.name, i.name, c.name from sys.tables t inner join sys.schemas s on t.schema_id = s.schema_id inner join sys.indexes i on i.object_id = t.object_id inner join sys.index_columns ic on ic.object_id = t.object_id inner join sys.columns c on c.object_id = t.object_id and ic.column_id = c.column_id where i.index_id > 0 and i.type in (1, 2) -- clustered & nonclustered only and i.is_primary_key = 0 -- do not include PK indexes and i.is_unique_constraint = 0 -- do not include UQ and i.is_disabled = 0 and i.is_hypothetical = 0 and ic.key_ordinal > 0 order by ic.key_ordinal which is not exactly what I want. What I want is to list all user-defined indexes (which means no indexes which support unique constraints & primary keys) with all columns (ordered by how do they apper in index definition) plus as much metadata as possible.

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  • SQL SERVER - Detecting non-indexed columns but used in WHERE clause

    - by Vadi
    How to detect a column included in WHERE clause but used in indexed? Little Background: Until the time the table has few number of records things will be okay, once it started having millions of records then index should be created for a column which is used in WHERE clauses in stored procs, inline queries etc., Since we have hundreds of stored procs and queries that often gets changed by the devs I wanted to have a automated way of identifying those columns that are used in WHERE clauses but not an index is created. How to do that in SQL SERVER 2008?

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  • jQuery - Finding the element index relative to its container

    - by Hary
    Here's my HTMl structure: <div id="main"> <div id="inner-1"> <img /> <img /> <img /> </div> <div id="inner-2"> <img /> <img class="selected" /> <img /> </div> <div id="inner-3"> <img /> <img /> <img /> </div> </div> What I'm trying to do is get the index of the img.selected element relative to the #main div. So in this example, the index should be 4 (assuming 0 based index) and not 1. My usual way to go about getting indexes is using $element.prevAll().length but, obviously, that will return the index relative to the #inner-2 div. I've tried using $('img.selected').prevAll('#main').length but that's returning 0 :/

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  • Multi-variable indexes in postgres

    - by Jackson Davis
    Im looking at an application where I will be doing quite a few SELECTs where I am trying to find column_a = x AND column_b = y. Is the correct to create that index that something like the following? CREATE INDEX index_name ON table (column_a, column_b)

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  • compare two following values in numpy array

    - by Billy Mitchell
    What is the best way to touch two following values in an numpy array? example: npdata = np.array([13,15,20,25]) for i in range( len(npdata) ): print npdata[i] - npdata[i+1] this looks really messed up and additionally needs exception code for the last iteration of the loop. any ideas? Thanks!

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  • A question about indexes regarding to the gain of inserts & updates in database

    - by Mestika
    Hi, I’m having a question about the fine line between the gain of an index to a table there is growing steadily in size every month and the gain of queries with an index. The situation is, that I’ve two tables, Table1 and Table2. Each table grows slowly but regularly each month (with about 100 new rows for Table1 and a couple of rows for Table2). My concrete question is whether to have an index or to drop it. I’ve made some measurement that an covering index on Table2 improve my SELECT queries and some rather much but again, I’ve to consider the pros and cons but having a really hard time to decide. For Table1 it might not be necessary to have an index because the SELECT queries there is not that common. I would appreciate any suggestion, tips or just good advice to what is a good solution. By the way, I’m using IBM DB2 version 9.7 as my Database system Sincerely Mestika

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  • How to index a date column with null values?

    - by Heinz Z.
    How should I index a date column when some rows has null values? We have to select rows between a date range and rows with null dates. We use Oracle 9.2 and higher. Options I found Using a bitmap index on the date column Using an index on date column and an index on a state field which value is 1 when the date is null Using an index on date column and an other granted not null column My thoughts to the options are: to 1: the table have to many different values to use an bitmap index to 2: I have to add an field only for this purpose and to change the query when I want to retrieve the null date rows to 3: locks tricky to add an field to an index which is not really needed What is the best practice for this case? Thanks in advance Some infos I have read: Oracle Date Index When does Oracle index null column values?

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  • Creating an appropriate index for a frequently used query in SQL Server

    - by Slauma
    In my application I have two queries which will be quite frequently used. The Where clauses of these queries are the following: WHERE FieldA = @P1 AND (FieldB = @P2 OR FieldC = @P2) and WHERE FieldA = @P1 AND FieldB = @P2 P1 and P2 are parameters entered in the UI or coming from external datasources. FieldA is an int and highly on-unique, means: only two, three, four different values in a table with say 20000 rows FieldB is a varchar(20) and is "almost" unique, there will be only very few rows where FieldB might have the same value FieldC is a varchar(15) and also highly distinct, but not as much as FieldB FieldA and FieldB together are unique (but do not form my primary key, which is a simple auto-incrementing identity column with a clustered index) I'm wondering now what's the best way to define an index to speed up specifically these two queries. Shall I define one index with... FieldB (or better FieldC here?) FieldC (or better FieldB here?) FieldA ... or better two indices: FieldB FieldA and FieldC FieldA Or are there even other and better options? What's the best way and why? Thank you for suggestions in advance!

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  • mysql query and index

    - by parm.95
    Does this query will be faster with a index on "t.type1" and "x.type1" or only index on "x.type1" is enought? SELECT t.id, x.id FROM t INNER JOIN x ON x.type1=t.type1 WHERE t.id=1

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  • iphone indexed table view problem

    - by steveY
    I have a table view in which I'm using sectionIndexTitlesForTableView to display an index. However, when I scroll the table, the index scrolls with it. This also results in very slow refreshing of the table. Is there something obvious I could be doing wrong? I want the index to remain in place on the right while the table scrolls. This is the code I'm using for the index titles: - (NSArray *)sectionIndexTitlesForTableView:(UITableView *)tableView { NSMutableArray *tempArray = [[NSMutableArray alloc] init]; [tempArray addObject:@"A"]; [tempArray addObject:@"B"]; [tempArray addObject:@"C"]; [tempArray addObject:@"D"]; ... return tempArray; }

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  • Approach for altering Primary Key from GUID to BigInt in SQL Server related tables

    - by Tom
    I have two tables with 10-20 million rows that have GUID primary keys and at leat 12 tables related via foreign key. The base tables have 10-20 indexes each. We are moving from GUID to BigInt primary keys. I'm wondering if anyone has any suggestions on an approach. Right now this is the approach I'm pondering: Drop all indexes and fkeys on all the tables involved. Add 'NewPrimaryKey' column to each table Make the key identity on the two base tables Script the data change "update table x, set NewPrimaryKey = y where OldPrimaryKey = z Rename the original primarykey to 'oldprimarykey' Rename the 'NewPrimaryKey' column 'PrimaryKey' Script back all the indexes and fkeys Does this seem like a good approach? Does anyone know of a tool or script that would help with this? TD: Edited per additional information. See this blog post that addresses an approach when the GUID is the Primary: http://www.sqlmag.com/blogs/sql-server-questions-answered/sql-server-questions-answered/tabid/1977/entryid/12749/Default.aspx

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  • How to get Google Search Query Kind of Content Indexed ?

    - by Sunil
    Hello: I was looking for something like flash grabber and I came across this result http://www.giveawayoftheday.com/flash+grabber+firefox/ I am just wondering how giveawayoftheday has managed to get "flash+grabber+firefox/" indexed in Google ? Did they submit via Google Sitemap or is Google doing this on its own ? Or something else ? Kindly enlighten. Thanks.

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