Re: Postgresql jsonb

On 15 August 2015 at 00:09, Deepak Balasubramanyam <deepak.balu@gmail.com> wrote:

Hi,

I have a table (20 million rows) in Postgresql 9.4 that contains a bigint id as the primary key and another column that contains jsonb data. Queries run on this table look like so...

------------

## Query

------------

select ... from table

WHERE table.column ->'item'->> 'name' = 'value'

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I'd like to make an effort to get Postgresql to keep all data available in this table and any index on this table in memory. This would ensure that sequence or index scans made on the data are fairly fast.

Research into this problem indicates that there is no reliable way to get Postgresql to run off of RAM memory completely (http://stackoverflow.com/a/24235439/830964). Assuming the table and its indexes amount to 15 gb of data  on the disk and the machine contains 64GB of RAM with shared buffers placed at anywhere from 16-24 GB, here are my questions...

1. When postgresql returns data from this query, how can I tell how much of the data was cached in memory?

It depends which memory you're talking about. If you mean pages that are in the shared buffers then you can just

EXPLAIN (ANALYZE, BUFFERS) select ... from table;

You'll see Buffers: shared read=N if any buffers were "read from disk" but keep in mind they still might not be coming from disk, they could be cached by the operating system in memory.

Regards

David Rowley

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