# PostgreSQL Backend Flowchart

## by Bruce Momjian

A query come into the backend via data packets coming in through TCP/IP and Unix Domain sockets. It is loaded into a string, and passed to the parser, where the lexical scanner, scan.l, breaks the query up into tokens(words). The parser uses gram.y and the tokens to identify the query type, and load the proper query-specific structure, like CreateStmt or SelectStmt.

The query is then identified as a Utility function or a more complex query. A Utility query is processed by a query-specific function in commands. A complex query, like SELECT, UPDATE, and DELETE requires much more handling.

The parser takes a complex query, and creates a Query structure that contains all the elements used by complex queries. Query.qual holds the WHERE clause qualification, which is filled in by transformWhereClause(). Each table referenced in the query is represented by a RangeTableEntry, and they are linked together to form the range table of the query, which is generated by makeRangeTable(). Query.rtable holds the queries range table.

Certain queries, like SELECT, return columns of data. Other queries, like INSERT and UPDATE, specify the columns modified by the query. These column references are converted to Resdom entries, which are linked together to make up the target list of the query. The target list is stored in Query.targetList, which is generated by transformTargetList().

Other query elements, like aggregates(SUM()), GROUP BY, ORDER BY are also stored in their own Query fields.

The next step is for the Query to be modified by any VIEWS or RULES that may apply to the query. This is performed by the rewrite system.

The optimizer takes the Query structure and generates an optimal Plan, which contains the operations to be performed to execute the query. The path module determines the best table join order and join type of each table in the RangeTable, using Query.qual(WHERE clause) to consider optimal index usage.

The Plan is then passed to the executor for execution, and the result returned to the client.

There are many other modules that support this basic functionality. They can be accessed by clicking on the flowchart.

Another area of interest is the shared memory area, containing table data/index blocks, locks, backend information, and lookup tables for these structures:

- ShmemIndex - lookup shared memory addresses using structure names
- Buffer Descriptors - control header for buffer cache block
- Buffer Blocks - data/index buffer cache block
- Shared Buf Lookup Table - lookup of buffer cache block address using table name and block number( BufferTag)
- LockTable (ctl) - lock table structure, specifiying table, lock types, and backends holding or waiting on lock
- LockTable (lock hash) - lookup of LockTable structures using relation, database object ids
- LockTable (xid hash) - lookup of LockTable structures using transaction id, LockTable address
- Proc Header - information about each backend, including locks held/waiting, indexed by process id

Each data structure is created by calling ShmemInitStruct(), and the lookups are created by ShmemInitHash().

Click on an item to see more detail or click to see the full index.