CREATE INDEX name ON table USING GIN (column);

Example:

postgres=# create table ts(doc text, doc_tsv tsvector);

postgres=# insert into ts(doc) values
  ('Can a sheet slitter slit sheets?'),
  ('How many sheets could a sheet slitter slit?'),
  ('I slit a sheet, a sheet I slit.'),
  ('Upon a slitted sheet I sit.'),
  ('Whoever slit the sheets is a good sheet slitter.'),
  ('I am a sheet slitter.'),
  ('I slit sheets.'),
  ('I am the sleekest sheet slitter that ever slit sheets.'),
  ('She slits the sheet she sits on.');

postgres=# update ts set doc_tsv = to_tsvector(doc);

postgres=# create index on ts using gin(doc_tsv);

ginbuild
{
    ...
    // 初始化工作，如 创建 gin 索引的 meta 和 root，即 XLOG 等
    buildInitialize(index, &buildstate);
    // scan heap tuples 调用 ginBuildCallback 处理每个要加入索引的 tuple
    // ginBuildCallback 会从 heap tuple 中提取 entries，如果有多个值
    // 会对这些值进行去重和排序。得到去重及排完序的 entries 后，调用 ginInsertBAEntries
    // 将这些 entries  及 对应的 tids 插入一棵RB-tree
    reltuples = tableam_index_build_scan(heap, index, indexInfo, false, ginBuildCallback, (void*)&buildstate);
    ...
    // 从RB-tree中把之前插入的 entries 和 tids scan 出来，插入到 gin index 中
    while ((list = ginGetBAEntry(&buildstate.accum, &attnum, &key, &category, &nlist)) != NULL) {
        /* there could be many entries, so be willing to abort here */
        CHECK_FOR_INTERRUPTS();
        // 如果 key 不存在，则新增一个 key entry，如果已经存在则更新对应的 tids
        // 首先在gin索引中查找到对应 key 的叶子节点，如果 key 已经存在，更新对应的 tids
        // 不存在则插入一个新的叶子节点
        ginEntryInsert(&buildstate.ginstate, attnum, key, category, list, nlist, &buildstate.buildStats);
    }

    ...
    // 更新 meta-page 中的信息, 记 XLOG
    ginUpdateStats(index, &buildstate.buildStats);
    ...
    返回结果
}

void ginEntryInsert(GinState *ginstate, OffsetNumber attnum, Datum key, GinNullCategory category,
                    ItemPointerData *items, uint32 nitem, GinStatsData *buildStats)
{
    GinBtreeData btree;
    GinBtreeEntryInsertData insertdata;
    GinBtreeStack *stack = NULL;
    IndexTuple itup;
    Page page;

    insertdata.isDelete = FALSE;

    /* During index build, count the to-be-inserted entry */
    if (buildStats != NULL)
        buildStats->nEntries++;

    ginPrepareEntryScan(&btree, attnum, key, category, ginstate);

    // 在 B-tree 中找到叶子节点
    stack = ginFindLeafPage(&btree, false);
    page = BufferGetPage(stack->buffer);

    // 如果 key 已经存在
    if (btree.findItem(&btree, stack)) {
        /* found pre-existing entry */
        itup = (IndexTuple)PageGetItem(page, PageGetItemId(page, stack->off));
        // 如果是 posting tree 结构
        if (GinIsPostingTree(itup)) {
            /* add entries to existing posting tree */
            BlockNumber rootPostingTree = GinGetPostingTree(itup);

            /* release all stack */
            LockBuffer(stack->buffer, GIN_UNLOCK);
            freeGinBtreeStack(stack);

            /* insert into posting tree */
            ginInsertItemPointers(ginstate->index, rootPostingTree, items, nitem, buildStats);
            return;
        }
        // 如果是 posting  list
        /* modify an existing leaf entry */
        itup = addItemPointersToLeafTuple(ginstate, itup, items, nitem, buildStats);

        insertdata.isDelete = TRUE;
    } else { // 对应的 key 不存在， 需要新建一个叶子节点里的对象
        /* no match, so construct a new leaf entry */
        itup = buildFreshLeafTuple(ginstate, attnum, key, category, items, nitem, buildStats);
    }

    /* Insert the new or modified leaf tuple */
    insertdata.entry = itup;
    ginInsertValue(&btree, stack, &insertdata, buildStats);
    pfree(itup);
    itup = NULL;
}

// 用于表示一个 key 及 与其关联的 tids 的数据结构
typedef struct GinEntryAccumulator {
    RBNode rbnode;
    Datum key;
    GinNullCategory category;
    OffsetNumber attnum;
    bool shouldSort;
    ItemPointerData *list;
    uint32 maxcount; /* allocated size of list[] */
    uint32 count;    /* current number of list[] entries */
} GinEntryAccumulator;

// Gin 索引整体结构为 B-tree 结构
// B-tree 中的一个节点
typedef struct GinBtreeStack {
    BlockNumber blkno;
    Buffer buffer;
    OffsetNumber off;
    ItemPointerData iptr;
    /* predictNumber contains predicted number of pages on current level */
    uint32 predictNumber;
    struct GinBtreeStack *parent; // 父节点
} GinBtreeStack;

typedef struct GinBtreeData *GinBtree;