Postgres database development, table physical operation meaning __ database

SEQ Scan: Sequential scans, each read again
Index Scan: Indexed scan, read based on index conditions, suitable for less-hit queries, with sequential results

Hash: Find the hash value of the result set for subsequent hash links.
Hash connection: According to the hash value, the conditional connection.
Nest connections: Nested links, t1 tables correspond to rows without a T2 table, such as inner Join,cross join available
Merge connection: Also called merge Simi join merging joins, merges a semi join, which must first sort the columns of the two table join conditions, look for the hit row in the T2 table based on the minimum value of the T1 table, and, if not, discard the minimum value in the T1, and proceed to the next round, until the end, This process often operates in a database using temporary tables, which are very fast, but have a long time to sort.
Sort: Sorting

The nest connection can get results immediately, and the hash may get results immediately, and the merge can only output results after the entire connection has ended.

Meterialize: materialization, this intention: Save time-consuming query operation results, save time cost, is a space in exchange for time operation, materialized view, understanding of the word, if a query two times to use a table such as T, if two query conditions are not the same, then, Each query for T can be viewed as a view of T, although no query ***view or CREATE view can be seen in table operation, so the so-called create view is just a query of the condition is too long, this method to save the process of this query , next time, go straight to it.

Bitmap index: Bitmap index scan used for a column to establish an index representing its value, scanned by this bit to determine whether to read.

Bitmap heap Scan: The conditional processing after a bitmap sweep table is used for merging bitmap index scan results.

Initplan: This is to make a self-examination search, by $ to get its return value, to provide the basis for subsequent inquiries, when the subquery has no external dependencies, will use this, and only do once, when there is external dependence, will use Subplan.

Subplain: This is a subquery, the corresponding external query is not a line, will do it again.