Questions tagged [explain]

The EXPLAIN statement can be used to obtain information about the database executes a statement.

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1answer
27 views

Does a huge key length value for a mulibyte column affect the index performance?

When I look at the EXPLAIN results, the key len value is always calculated based on the actual column length multiplied on the maximum number of bytes for the chosen encoding. Say, for a varchar(64) ...
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49 views

SQL: ON vs. WHERE in SEMI JOIN

Considering the following two (equivalent) SQL statements. Why do their execution plan differ (especially in performance) that much? I'm using Postgres 11.9 in this example. select p.name from person ...
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20 views

UPDATE vs SELECT returns different Execution Plan

In my DB table, where ID is the PK, if I use this: EXPLAIN SELECT * FROM table WHERE ID = 3; The type and the ref columns of the explain output table are both equal to const. Now, when I use this: ...
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1answer
33 views

Interpreting startup time and varying plans for Postgres seq scans

In asking a recent question, some mysterious startup time components came up in my EXPLAIN ANALYZE output. I was playing further, and discovered that the startup time drops to near 0 if I remove the ...
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1answer
25 views

how to sum times in EXPLAIN for Postgres parallel query

I'm having trouble understanding this EXPLAIN on Postgres 12.3: EXPLAIN (ANALYZE, VERBOSE, BUFFERS) SELECT count(1) FROM mytable WHERE page ~ 'foo'; This is a 22GB table with 30 million rows, on a ...
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32 views

Postgresql - extremely slow basic SELECT query

I have a materialized view with about 1.6 million entries that gets refreshed every hour or so. I have a && and ANY() as the filters in the query. I observed the query was taking too long (...
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17 views

Understanding rows_examined_per_scan and rows_produced_per_join

I'm not mysql guy, but I've got opportunity to work with that recently - I've got to optimize some query on Mysql5.7 (I have to cover that also on 5.6, but started with 5.7 as it has apparently more ...
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1answer
26 views

Read explain results from oracle database

hi currently i have query like this on oracle database 12 Query Detail because script is too long if input here the query is too long execute time from explain results i got this PLAN_TABLE_OUTPUT ...
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1answer
34 views

See content of temporary Table in Oracles' EXPLAIN

I'm executing a Query in Oracle SQL Developer and eventually call SELECT * FROM TABLE ( dbms_xplan.display ); to get an impression what Oracle just did. There's a column Name in the output that ...
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17 views

Analyzing performance of an insert int postgres

I have a statement which is inserting a bunch of rows (doesn't really matter what or where for this question) into a Postgres DB and it isn't as fast as I'd like. I can run an explain query to see ...
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1answer
34 views

How is the value of row column is estimated in EXPLAIN statement in mysql?

I have the following setup: create table Test(id integer, itemID integer, title varchar(100) ,PRIMARY KEY (id, itemID)); insert into Test(id, itemID, title) values(1,678, "Hello"); insert ...
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27 views

Extreme slow query for a simple indexed selection - Why? [duplicate]

i have a table in my database that is described as follows: -- Table: public.devicemeasurement_mqtt -- DROP TABLE public.devicemeasurement_mqtt; CREATE TABLE public.devicemeasurement_mqtt ( ...
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1answer
19 views

Why is MySQL using this index? Can I force it to use another?

I have a query, and when I call EXPLAIN for it, it appears that my db is using an unhelpful index and ignoring a useful index. EXPLAIN SELECT id, full_name, agent_id, brand_id FROM `mytable` WHERE ( ...
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1answer
28 views

PostgreSQL is skipping indexes when using `order by` + `limit`

I have a postgreSQL database with two big tables, connections and sessions select "c"."id" "id", "ch"."title" "channel", "ch".&...
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72 views

What is Oracle's Window Buffer Operator?

Oracle database's explain plan sometimes includes Window (Buffer). What does this operator do? Here is the SQL. The primary keys are RECORD_DATE, TYPE and KEY. select source.RECORD_DATE, ...
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1answer
37 views

MySQL read explain - using where

In Oracle there is an important note in the explain plain where it says filter a.col1=... this one letting you know col1 is first retrieved from the disk and only then filter is been done. This ...
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1answer
27 views

Optimising query: not sure why LOWER() index isn't used

I've got comments that have 1..N comment_notifications. Each notification basically contains a username of the user notified. Each notification can be resolved_at when the user is done with it. This ...
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2answers
37 views

How can I see execution time of an ALTER TABLE statement?

Is it possible to see the execution time of an alter table command? I think with \timing on in psql, it shows more than the execution time, but also the client round trip, and maybe other things as ...
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24 views

Select Type UNION RESULT doesn't appear in the output of Explain statement

EXPLAIN SELECT actor_id, (SELECT 1 FROM sakila.film_actor WHERE film_actor.actor_id = der_1.actor_id LIMIT 1) FROM ( SELECT actor_id FROM sakila.actor LIMIT 5 ) AS der_1 UNION ALL SELECT film_id, (...
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1answer
129 views

How to optimize Merge Anti Join?

While trying to find the fastest way to do a bulk update of a large table, I came up with the following plan: having main_table (potentially above 1000 mln rows) and update table up to a few million ...
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2answers
77 views

Postgres 10.6: Why is a particular partition's index not being used?

I'm truly stumped as to what's causing this issue and hoping for some guidance. Firstly, this is in our STAGE environment and we partition this table based on the createdTime every month. I have ...
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1answer
21 views

How to get the result of MySQL explain query into a file?

I am unable to get the results of the explain query in its normal tabular format or json format into a file. select * from table_name into outfile 'file.format'; select trace from information_schema....
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1answer
197 views

Very slow postgresql aggregation

EDIT: docker image for testing with the same data (obfuscated) $ docker run --rm --name pg -d homer5439/pgagg $ docker exec -ti pg bash # createdb -U postgres test; zcat /tmp/corr.sql.gz | psql -U ...
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1answer
47 views

Explain shows index doesnt working but i have an index

The query in MySQL is: EXPLAIN EXTENDED SELECT id, username, name, last_name, IF(friend IS NOT NULL,1,0) as friend, activated, if(private=1 AND IF(friend IS ...
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1answer
120 views

MYSQL Select query stuck at “Sending data”

I have an select query with about 1M records, I'm working on Magento 1.9 database. SELECT IF(sup_ap.is_percent = 1, TRUNCATE(mt.value + (mt.value * sup_ap.pricing_value / 100), 4), mt....
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212 views

MongoDB Explain plan - elapsed time breakdown for executionStats

I have run an explain plan for a query, and am a bit confused with the timings of each stage, and how they aggregate to the total elapsed time. Following command was run to generate the statistics, ...
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1answer
28 views

How to correctly interpret index usages from EXPLAIN of mysql?

I've exported the results of an EXPLAIN run on a query. What I find confusing is that there's the key column listing out one of the indexes from the list of possible_keys(not shown in picture) however ...
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66 views

How to trigger Mysql parallel query function to start?

Please help,how to trigger Mysql parallel query function to start? MySQL version is 8.0.19. mysql> show variables like 'innodb_parallel_read_threads'; +------------------------------+-------+ | ...
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1answer
32 views

How to run analysis on a 16M row table faster

I have two tables students with 6k records and transactions with 16M records. I have a query similar to this: select section, count(s.students) from students as s where upper(name) in (select ...
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1answer
580 views

Postgres EXPLAIN ANALYZE is super slow

there is Postgres 9.6 on Redhat I want to EXPLAIN ANALYZE slow query (quite complex one) and it takes too much time. My session with EXPLAIN ANALYZE consume 100% of one CPU and doesn't finish even ...
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3answers
1k views

MySQL DELETE statement doesn't use index although the same SELECT query does

I've got a table with ~30 million rows (and soon twice/triple times more) where I have to do quite regular updates. The table structure is like the following: id, cookie_id VARCHAR(45), country ...
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1answer
135 views

How to profile the PostgreSQL query planner diagnose what is causing it to take up large amounts of memory and time

I'm fighting with a specific "generated" query (the SQL is generated from a proprietary system, and I won't be able to share it or the execution plan here), and running into simply massive memory ...
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1answer
283 views

Performance issues with CAST function

Observing this performance issue that can't be explained with EXPLAIN in a PostgreSQL 9.6 database. There is currently a table public.users, which has a cast on it to output a record to JSONb. (e.g. ...
2
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1answer
194 views

Time Difference Between actual total time and execution time in EXPLAIN ANALYZE

I have the following output of EXPLAIN ANALYZE query on PostgreSQL 11.5 EXPLAIN (ANALYZE, VERBOSE, BUFFERS) SELECT actor.actor_id, actor.first_name, act.num FROM actor INNER JOIN ( SELECT ...
1
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1answer
130 views

MySQL uses range and where on UPDATE with the primary key

I have these simple SELECT and UPDATE requests on MySQL using the primary key, there are about 13 million rows in the table. When running an EXPLAIN on both, I notice that the UPDATE one does not use ...
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0answers
27 views

How to index on joined tables

Total table_parent rows: 1318 Total table_child rows: 12 mil++ EXPLAIN select SQL_CALC_FOUND_ROWS table_child.* , table_parent.name AS parent_name from `table_child` inner join `table_parent` ...
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1answer
31 views

Why does performing a query on a second environment, it takes forever to run, and scans hundreds of millions of rows?

I have a query, it is fairly big, but not THAT big. One the baseline environment, it runs in less than 1/3 of a second. When we moved the database to RDS in Amazon, it takes 10-15 minutes to run. I ...
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1answer
33 views

How to optimize query based on explain analyze in PostgreSQL?

I am running the following query via EXPLAIN ANALYZE but it is taking a bit of a long time to run. Based on the output of EXPLAIN ANALYZE, what are some things I can do to my table to make this query ...
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2answers
435 views

PostgreSQL 10 Optimizing slow query performance

I have the following query: select i.id as id, i.ts_updated as updated, i."name" as name, cast(attr.data_norm as text) as data_norm, cast(_59.json_agg as text) as _59, ...
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1answer
811 views

Help understanding mongodb explain results

I have an index on field f1. {f1:1} Running an explain on the following query db.coll.find() says that the winning plan is a CollScan (expected) However, for db.coll.find().sort({f1:1}) the ...
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3answers
210 views

MySQL why EXPLAIN result is confusing?

Below is my Query: select * from ( SELECT * from Player where Country_Name in (1,2) ) as A inner join Country C ON A.Country_Name=C.Country_Id; ...
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2answers
413 views

Postgres Row Level Security policy optimizes poorly compared to inline version

I have a query which looks like this: SELECT post.id, post.author_id, post.published_at, post.content FROM post WHERE post.group_id = 1 ORDER BY post.published_at DESC, post.id LIMIT 5; ...
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0answers
27 views

Is it possible to replace count distinct inside Correlated sub-queries?

I'm trying to optimize the query bellow. The execution Plan is showing multiple sorting before aggregations (I think this is due to the count distincts in the correlated sub-queries)... Is it ...
0
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1answer
136 views

how to create an explain plan for the below query? I'm getting ORA-00900: invalid SQL statement error

I'm getting ORA-00900: invalid SQL statement error when i tried. but the sql query is running fine without explain. Query: create or replace view TaskView as select T1.PersonID,T2.ID ,T2.Task from ...
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0answers
107 views

explain on view shows derived while explain on select shows simple select type

I have a view v which involves a couple of sub-selects in its SELECT clause plus a couple of joins by id or indexed column. The view query is like: SELECT col1, col2, (SELECT ... FROM subt) AS col3 ...
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0answers
53 views

Different operations in different databases

So I have just executed EXPLAIN PLAN FOR on the same query in two databases that I've been told are identical in every regard except for slight variations in data amount. And the result of it seems ...
3
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1answer
1k views

Does SQL Server Management Studio 2017 Contain a New Explain Feature

TL;DR I happened upon a feature in SQL Server Management Studio v17.9 which I was not aware of. It looks a bit like the EXPLAIN found in Oracle, PostgreSQL and MySQL. Where is this feature documented?...
3
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1answer
332 views

Why does a SELECT (JOIN on regex) gets 30x slower with +1 row (query plan stays the same)? Lack of memory?

Here is my query, it takes 2.7 sec: SELECT t2.id, t1.id FROM sample t1 INNER JOIN (select * from regex_set limit 33) t2 ON t1.data ~* t2.regex; The associated EXPLAIN (ANALYSE,BUFFERS) output: ...
4
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1answer
795 views

MySQL Workbench Visual Explain not working with MariaDB 10.3.9

I recently installed MySQL Workbench 8.0.12 and MariaDB 10.3.9. I created my database and want to start fine tuning indexes and views. I can run a query with the EXPLAIN button and get the results in ...
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1answer
108 views

Postgres logging_collector probably blocks auto_explain output

Postgres 10.4 Postgres 9.6 Problem on both versions I want to collect postgresql slow queries plan, so I added to my postgresql.conf setting from documentation session_preload_libraries = '...