Is there a way to get an exact count from Postgres statistics tables without running vacuum, which is also a costly operation? It seems there is currently no built-in way to do what you require in PostgreSQL. People are working toward such capabilities.
While nobody can say with any certainty when such features will make it into a PostgreSQL release, I think it's safe to predict that it will not be before lateand most probably later than that. Meanwhile, you could manually implement a solution using triggers; for example as described in Postgresql General Bits by A.
Elein Mustain. The idea is to maintain an always-current row count in a separate table using triggers. Be aware that this may add significant overhead to data modifications.
You might want to consider using the built-in function pgstattuple which is doucmented as follows:. It does this by skipping pages that have only visible tuples according to the visibility map if a page has the corresponding VM bit set, then it is assumed to contain no dead tuples. For such pages, it derives the free space value from the free space map, and assumes that the rest of the space on the page is taken up by live tuples. For pages that cannot be skipped, it scans each tuple, recording its presence and size in the appropriate counters, and adding up the free space on the page.
Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. Getting the exact count of rows in Postgres database Ask Question. Asked 2 years, 1 month ago.
Active 2 years, 1 month ago. Viewed times. Is there any way to get the exact row count of all tables in Postgres in fast way? Priya Priya 1 1 silver badge 3 3 bronze badges. Active Oldest Votes. A module that attempts to add more features to materialized views work still in progress as far as I know : github. You might want to consider using the built-in function pgstattuple which is doucmented as follows: The pgstattuple module provides various functions to obtain tuple-level statistics.
John aka hot2use John aka hot2use Yes, the mechanism is the same. However, seeing as pgstattuple acquires only a read lock on the relation.
So the results do not reflect an instantaneous snapshot; concurrent updates will affect them. This is something you would have to test against your large table. Sign up or log in Sign up using Google.Last week, I had a requirement to check the row count of all tables having a specific schema of databases. Here i will explain how to get row counts of all:. We can get total row counts of all tables using system catalog view sys. Read attached article if you are getting wrong row counts using this DMV or catalog view.
System catalog view sys. All tables and indexes in SQL Server contain at least one partition, whether or not they are explicitly partitioned. Run below script to get the row count of all tables in a database. Run below scripts to get the row count of all heap tables. You can see there are only two tables that have no cluster index in above screenshot.
We can also get the row count of all tables having a specific schema by adding a condition in WHERE clause.
DMV sys. We will leverage this DMV to get row count details of all tables. Run below script to get the row count of all tables using this DMV. I hope you like this article. You can comment about your questions in comment section. Tags: dmv row count. February 15, January 15, December 6, Your email address will not be published.I used to use the selected answer above, but this is much easier. If you know the number of tables and their names, and assuming they each have primary keys, you can use a cross join in combination with COUNT distinct [column] to get the rows that come from each table:.
Poster wanted row counts without counting, but didn't specify which table engine. With InnoDB, I only know one way, which is to count. I am making no assertions about this other than that this is a really ugly but effective way to get how many rows exist in each table in the database regardless of table engine and without having to have permission to install stored procedures, and without needing to install ruby or php.
Yes, its rusty. Yes it counts. The entire result of the query shown here - all rows taken together - comprise a valid SQL statement ending in a semicolon - no dangling 'union'. The dangling union is avoided by use of a union in the query below.
Create and assign the list of tables to the array variable in this bash script separated by a single space just like in the code below. You can probably put something together with Tables table.
I'm not sure if this works with all versions, but I'm using 5. If you want the exact numbers, use the following ruby script. You need Ruby and RubyGems. This is what I do to get the actual count no using the schema It's slower but more accurate. It's a two step process at Get list of tables for your db. You can try this. It is working fine for me.
How to concatenate text from multiple rows into a single text string in SQL server? Get list of all tables in Oracle? Inserting multiple rows in a single SQL query?Join over 12K people who already subscribe to our monthly Citus technical newsletter. Our goal is to be useful, informative, and not-boring. Everybody counts, but not always quickly. This article is a close look into how PostgreSQL optimizes counting. If you know the tricks there are ways to count rows orders of magnitude faster than you do already.
The problem is actually underdescribed — there are several variations of counting, each with its own methods. First think whether you need an exact count or whether an estimate suffices. Next, are you counting duplicates or just distinct values? Finally do you want a lump count of an entire table or will you want to count only those rows matching extra criteria?SQL Table Row Count
Measuring the time to run this command provides a basis for evaluating the speed of other types of counting. Pgbench provides a convenient way to run a query repeatedly and collect statistics about performance. However the opposite is true. Historically the expression ought to have been defined as count. This means each transaction may see different rows — and different numbers of rows — in a table.
There is no single universal row count that the database could cache, so it must scan through all rows counting how many are visible. Performance for an exact count grows linearly with table size. As we double the table size the query time roughly doubles, with cost of scanning and aggregating growing proportionally with one other. How can we make this faster?
Something has to give, either we can settle for an estimated rather than exact count, or we can cache the count ourselves using a manual increasing-decreasing tally. However in the second case we have to keep a tally for each table and where clause that we want to count quickly later.
The following trigger-based solution is adapted from A. Elein Mustain. The speed of reading and updating the cached value is independent of the table size, and reading is very fast. However this technique shifts overhead to inserts and deletes. Without the trigger the following statement takes an average of 4. To do so we can lean on estimates gathered from PostgreSQL subsystems. Two sources are the stats collector and the autovacuum daemon. Andrew Gierth RhodiumToad advises:.There are even more choices to be made when you consider all the different operations and data structures available to Postgres.
For example, the documentation for EXPLAIN talks at some length about how the same information might be read differently based on what it will be used for and how much of it might be needed.
So making good row count estimates is core to what the query planner does. An example with four row…ers. Picture by Matteo Vistocco. It can be pretty hard to guess!
This is triggered by the autovacuum daemon, which is enabled by default. As we discussed earlier, the query planner uses row count estimates to choose between different query implementations with very different performance profiles.
If those estimates are a long way out, then the query planner can make some bad choices, leaving your query running very slowly indeed.
PostgreSQL COUNT Function
Just get a copy of the query plan. In pgMustard, we flag up instances where the row count estimates are out by a factor of 10 or more.
For example, you can force the query planner to use an index when it would otherwise have chosen a sequential scan. The query planner will be less able to adjust to changes in the structure or volume of your data, and may even be unable to take advantage of performance improvements in future versions of Postgres. As long as it has the right information, it will usually make the correct decisions.
If the statistics values are out of date, perhaps due to recent writes or deletes, then hopefully the refreshed values will be more accurate. Postgres 10 also introduced multivariate statistics. Normally, estimates assume that column values are independent, but naturally this is often not the case.
Suppose your query contains something like:. But if vegetarians are significantly more likely to prefer hummus than the population in general, then this number could be much higher. Photo by Ojashri Basnyat. Multivariate statistics are an attempt to solve this problem. Statistics are gathered on the columns as a group, so that Postgres can understand correlations or relationships between columns.
List tables by the number of rows in PostgreSQL database
The dark mode beta is finally here. Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. I need to know the number of rows in a table to calculate a percentage. If the total count is greater than some predefined constant, I will use the constant value. Otherwise, I will use the actual number of rows. But if my constant value isand I have 5, rows in my table, counting all rows will waste a lot of time.
I need the exact number of rows only as long as it's below the given limit. Otherwise, if the count is above the limit, I use the limit value instead and want the answer as fast as possible. Counting rows in big tables is known to be slow in PostgreSQL. To get a precise number it has to do a full count of rows due to the nature of MVCC.
There is a way to speed this up dramatically if the count does not have to be exact like it seems to be in your case. It is usually very close. It ignored the possibility that there can be multiple tables of the same name in one database - in different schemas. To account for that:. Faster, simpler, safer, more elegant. See the manual on Object Identifier Types. For example:. A bigger sample increases the cost and reduces the error, your pick.
Accuracy depends on more factors:. First, I need to know the number of rows in that table, if the total count is greater than some predefined constant. Postgres actually stops counting beyond the given limit, you get an exact and current count for up to n rows in the exampleand n otherwise.
Then examining the output with a regex, or similar logic. Depending on the complexity of your query, this number may become less and less accurate. In fact, in my application, as we added joins and complex conditions, it became so inaccurate it was completely worthless, even to know how within a power of how many rows we'd have returned, so we had to abandon that strategy.
But if your query is simple enough that Pg can predict within some reasonable margin of error how many rows it will return, it may work for you. In Oracle, you could use rownum to limit the number of rows returned. I am guessing similar construct exists in other SQLs as well. Reference taken from this Blog. If possible, changing the schema to remove duplication of text data. This way the count will happen on a narrow foreign key field in the 'many' table.
Again, this is to decrease the workload scan through a narrow column index. Your original question did not quite match your edit. Details about sys. Even faster but unreliable methods are detailed here. Learn more. Asked 8 years, 5 months ago.After all, it is a complicated query, and PostgreSQL has to calculate the result before it knows how many rows it will contain.
Yet if you think again, the above still holds true: PostgreSQL has to calculate the result set before it can count it. It is tempting to scan a small index rather then the whole table to count the number of rows. However, this is not so simple in PostgreSQL because of its multi-version concurrency control strategy. But this information is not redundantly stored in the indexes. To mitigate this problem, PostgreSQL has introduced the visibility mapa data structure that stores if all tuples in a table block are visible to everybody or not.
Maintaining such a row count would be an overhead that every data modification has to pay for a benefit that no other query can reap.
This would be a bad bargain. Moreover, since different queries can see different row versions, the counter would have to be versioned as well.
But there is nothing that keeps you from implementing such a row counter yourself. Suppose you want to keep track of the number of rows in the table mytable. You can do that as follows:. In that case you can use the estimate that PostgreSQL uses for query planning:. Obviously the only way to get an exact answer to this is to execute the query. In this article I want to explore the options you have get your result as fast as possible. Using an index only scan It is tempting to scan a small index rather then the whole table to count the number of rows.
Estimating query result counts Up to now, we have investigated how to speed up counting the rows of a table. He has been working with and contributing to PostgreSQL since